Introduction to Underworld Explorations

Characters almost always start their adventuring lives on the surface of their world, as they have to deal with the puzzles and problems around a hometown or similar environment. While many of these events take place above ground, it’s never that long before venturing into a cellar, some holding cells, or perhaps a hidden shrine, moves the adventurers on their first steps beneath the surface. Then, whatever these initial issues may appear to be, it usually comes to pass quickly that a party finds it needs to undertake a first mission entirely below ground.

Whatever that assignment might be, it will be different from adventuring under open skies. Whether pursuing kobolds or goblins that retreat to their own home, tracking zombies to a buried mausoleum, following a monster to its cave-like lair, climbing down the well in a cellar and unearthing a tunnel leading away from it, discovering the holding cells have a well-hidden secret door, or finding the altar in the shrine conceals a ladder descending into the earth, the path the adventurers have to follow will be daunting with its unfamiliarity and almost certainly put them at a disadvantage, at least at first.

But these ways into the subsurface realms are just the beginning. Soon, a party finds itself in environments created by unimaginable forces, whether natural, magical, or the result of concerted brute effort! Many such realms are the result of time and persistent energy, while others came about thanks to cataclysmic events and irresistible forces. These myriad underground settings are considered here.

Naturally, these excursions into unknown subterranean environs necessitate a greater degree of planning and choice of resources before even a single step can be taken. For what could be worse than to find yourself being hunted mercilessly by those very humanoids you were chasing, being buried alive by the undead who won’t suffer the same agonies, or getting lost hopelessly in a cave complex you haven’t made plans to escape from?

This site provides information, advice and guidance on a wide range of habitats, perils, unexpected and rare events, and unusual experiences, along with their possible boons and probable banes. There are thoughts on how to switch things up during these experiences, as well as ways to make the most of situations that would tax an unprepared or inexperienced party.

Anything that gives an adventurer an advantage as they come across colossal carved cities, far-reaching fungi forests, sweeping stygian seas, and yawning, immeasurable abysms is worth knowing, especially if it means the difference between legendary success or a lonely, soon-forgotten death.

Finally, in this brief introduction, what mustn’t be forgotten is that there is significantly more room to exist below the surface than on it. This means that despite the vast empty spaces and long uneventful journeys an adventurer might experience, the Underworld is an active, vibrant environment.

Most new adventurers are surprised that once below the surface, they find it is positively swarming with life of all kinds and bristling with many rare and strange creatures that are only too willing and able to part an unwary delver from a little property at best and their life at worst. This means that with food as frequently scarce and permanently bland as it can be, many denizens of the depths are quite prepared to harvest an Upperworlder for consumption, considering the flesh of those grown fat under a warming sun to be a true delicacy.

Now, with that caveat in mind, forge onwards intrepid reader, to discover what awaits those brave—some might say foolhardy—enough to plumb the sunless depths beneath their relatively safe surface-centered homelands.

The Surface and Below

The surface of a world, known as the pedosphere and meaning “sphere of soil”, forms great mountains or vast trenches and sits on top of the below-surface lithosphere, or “sphere of stone”. The crust is either oceanic, that is under water and starting anywhere between sea level and 11 km/6.9 miles below that on our own planet, or continental, which is above water and starting anywhere between at or just below sea level, and the tops of mountains here on Earth. This means that much of the dark world beneath the surface stretches high into the air, or is surrounded by vast oceans, often without those living in the “subterranean setting” being aware of what may be just feet away from them.

With such a mountain in a gaming world, piercing the surface and discovering the sky may not be problematic—unless sunlight is an issue—whereas weakening the lithosphere sufficiently for sea water to flood in is a significant concern; such an occurrence may change far-reaching volumes of the Underworld, completely altering the ecosystem in that region for many centuries to come as well as rendering it impossible to find drinkable water sources for many miles as the salt affects fauna and flora alike.

Of course, if these mountains aren’t significant enough for your gaming world, remember that our planetary neighbors have peaks that tower over those here on Earth: Maxwell Montes on Venus is considered to be 11 km/6.82 miles/36,000 feet high, while Mars has several known summits above this measurement, with the truly mighty Olympus Mons touching 21.17 km/13.15 miles/69,459 feet.

The constant rise and fall of an active planet’s surface means that some inland continental crust is below sea level but is protected from flooding by a ring of above-sea level land. These areas can be: lakes that hold water at least some of the time, with water levels changing seasonally, and even evaporating entirely temporarily; desert-like bowls that either may have been flooded or are sites where seismic activity has caused the lithosphere and pedosphere to drop significantly; or places where the walls of deep cracks and fissures have worn away to leave a significant flat surface.

Coming across such below-sea level spaces may reveal that they were part of the Underworld at some stage, but the surface formerly covering them has worn away by natural or other means, or perhaps was a material considered useful for building and has been taken away, such as deep peat deposits that originally contained tunnels. Part of their attraction may be that previously hidden information about a subterranean cult or shadowy species is now readily available for sages and adventurers to learn about; perhaps such field trips will unearth additional clues to other activities in which these creatures were engaged.

On Earth, the lithosphere, comprising the crust and upper-most solid mantle, is roughly 50–140 km/30–90 miles thick under oceans and 40–280 km/25–175 miles deep under land. Variably, the crust tends to be 5–75 km/3–45 miles thick, and the upper mantle 50–120 km/30–75 miles in depth. The combination of erosion and tectonic plate movement keep these figures consistent across the planet, with volcanic activity and earthquakes making significant if localized adjustments on an ongoing basis.

Beneath the lithosphere is the asthenosphere, or the “weak sphere”, a region of almost-solid flowing rocks anywhere between 80–200 km/50–120 miles below the surface. Less commonly, it is about 60 km/35 miles below the surface under the oceanic mantle and can even be just a few kilometers under the surface of, e.g., mid-ocean ridges, where it could be molten. It possibly extends as deep as 700 km/430 miles in places, but this potential limit is not well defined; suddenly entering such a layer in-game might well catch adventurers by surprise. This is not least because it is extremely hot, with the lithosphere-asthenosphere boundary considered to be at the point where the surrounding temperature is 1300 °C/2372 °F. It is here that rock turns from being rigid to ductile, and this part of a planet’s mantle forms, deforms, and then reforms constantly into new configurations. It is involved with plate tectonic movement amongst other geological activity, all of which a party will have to deal with at some point during a foray into the Underworld.

Beneath the asthenosphere (also thought of as the upper mantle) is the upper-lower mantle transition zone, which is roughly 410–660 km/250–410 miles thick, before becoming the lower mantle which is as deep as 2900 km/1800 miles below the surface. Rocks here are plastic, flowing relatively freely due to the higher viscosity. Temperatures at the base of the mantle, where it touches the outer core, are around 4000 °C/7232 °F.

In a game, this can be a strange and fascinating region with links to assorted demi-planes and quasi-elemental regions. Hybrid creatures abound: for example, while water may be something’s natural element, fire forever impinges on their lives, while earth and air attempt to force water out of the picture in the Underworld whenever it is possible to do so. Once deep below, explorers can happen upon all manner of elemental encounters without an easy means of escaping them. Preparation built around experience and magical expertise will undoubtedly be based on what takes place on the surface world, where the elements, however impressive, rarely demonstrate their full power or potential. In the Underworld, the four main elements can be seen to actively compete against one another as their strengths and weaknesses are tested in a manner rarely observed above ground.

A range of hazards present in transitory regions can be found in Complex Hazards: Elemental Boundaries and Beyond.

An Introduction for the Party

While metal of course provides significant defense and the best method of attack for some adventurers, for others it is the bane of their lives, hindering them at best and becoming entirely detrimental at worst. The four other factors are equally important, if not more so, when delving. Let us move on.

Without meals of some kind, a party is quickly going to perish. It’s true that a lack of water will kill you even more quickly—an average human can go without water for about three days before starting to suffer—but not having anything to eat for a few weeks can do the same thing. The Streamlined Survival Rules and the Foraging and Hunting Rules help cover these aspects.

Next, we consider medication. Whether simple first aid, removing toxins, overcoming an acute condition, maintaining a generally healthy disposition, or providing instant respite from life-threatening injury. This aspect is traditionally handled by potions and divine magic, but there is much merit in quizzing Underworld denizens about how they use fungi, other local flora, and unique fauna to compliment these two factors.

We must also consider movement. Unlike the surface, traveling in the Underworld is constantly in three dimensions, much as it is when you are underwater. However, gravity plays a much more significant role than if you are facing a lake of any depth, e.g. when the only route out of a cavern is a 100-ft.-deep chimney, perhaps made recently by the purple worm you have just dispatched after it rose through the floor. Therefore, the skill of moving in multiple directions safely includes, but is not limited to, being able to: climb and rappel alone and set ropes for others to do the same; recognize the best spots to place pitons for specific tasks as well as spikes used in an emergency; belay party members and gear that may weigh significant amounts; jump and swing across gaps after successfully using a grappling hook; build impromptu bridges using ropes; properly rope a party together with quick-release knots in case of the direst of emergencies if someone falls; squeeze through horizontal and vertical gaps, both with light and in the dark; swim into the unknown after ensuring a plan to return is in place; hold your breath for extended periods, whether under water or to avoid the surrounding air for some reason; and move silently, not least when many Underworld creatures have forsaken vision over the millennia in exchange for more effective senses.

On the surface, there is almost always “another way around” an impediment to movement. But below ground, it’s quite possible that there is JUST ONE route, depending on the theory you use to shape your Underworld. For the curious, these theories will be discussed in The Underworld as a Whole. Finally, we must always bear in mind how we overcome mysteries. There is no one set way to do this, other than almost near-universal acknowledgment that dealing with an unexpected encounter more often than not has to involve brains instead brawn. Decisions based on similar experience, use of lore and legend, turning one’s aptitude at a craft or profession to a new and innovative use, employing arcane skills, or talking through a situation via a common language will save time, energy and resources.

So, you’re planning to embark on a long expedition through the Underworld? Months, you say? Maybe even a year or more for you to achieve your goal? You’ve come to the right man, once again.

For even more importantly, remember to go forth with boldness, with confidence, with the type of swagger for which we dwarves are known! Your lack of fear will only kindle terror in the hearts of your enemies. An excess of caution on your part may give them courage, causing them to deem you timid and afraid. Do not let them think this of you! Act, act again, and if you have to stop and decide how you will all make progress, ensure you are ready to take action a third time while others dither. I say to you, stride boldly, looking neither left nor right so long as you can see the path before you. Let weaker companions watch your rear—you should have eyes only for the goal lurking somewhere in the darkness before you, for I can think of nothing in the Underworld that can overcome unbounded grit and determination! And always remember: strong drink, as fine a vintage as possible, is the “medicine of the darkness” for those who need their fortitude bolstered and their fears put out of mind!

The Environment

By its nature, adventuring involves delving into places that are dark, dangerous, and full of mysteries to be explored. The rules in this section cover some of the most important ways in which adventurers interact with the environment in such places. Let us recap the basics before we delve further.

Passive Perception

When you hide, there is a chance someone will notice you even if they are not searching. To determine whether such a creature notices you, the GM compares your Dexterity (Stealth) check with that creature’s passive Perception score, which equals 10 + the creature’s Wisdom modifier, as well as any other bonuses or penalties. If the creature has advantage, add 5. For disadvantage, subtract 5. For example, if a 1st-level character (with a proficiency bonus of +2) has a Wisdom of 15 (a +2 modifier) and proficiency in Perception, he or she has a passive Perception of 14. What Can You See? One of the main factors in determining whether you can find a hidden creature or object is how well you can see in an area, which might be lightly or heavily obscured, depending on the environment. An important distinction to note: in complete darkness creatures with darkvision can see as if there were dim light, which causes Wisdom (Perception) checks relying on vision to be at disadvantage. For more nuanced takes on light and darkness, see Light & Darkness.

Movement and Distances

On the surface, movement is relatively uncomplicated to control, as characters tend to move either straight forward, backward or sideways, or go across squares at a 45-degree angle. In the Underworld the terrain is different, and slopes are all around a party as historic water followed the path of least resistance. Table 2-1 shows how far a character moves horizontally and vertically along slopes of a variety of angles depending on their usual speed. It can be used also to calculate the actual distance of a switchback-style path combining various distances and angles.

These distances are based on an adventurer acting with some care and attention to the environment around them. If they choose to move more swiftly or, for example, fire a missile weapon without taking a considered aim, it maybe that they travel a little further downhill or a little less if moving uphill, or that a missile may hit a target at slightly greater distance if they are below the attacker but not quite reach a potential victim who is above the aggressor.

In addition, moving up or down a slope may make certain actions more or less challenging: keeping balance running downwards may incur a penalty or be attempted with disadvantage if the character isn’t dexterous, while getting to full charging speed takes a little longer if attempted going uphill. Going uphill may still be difficult for clumsy characters, but getting up to speed would be easier going downhill.

It is also worth bearing in mind that in real life, some creatures are more vulnerable when moving in certain directions. For example, animals that rely on speed to escape tend to be less efficient when fleeing downhill, while predators chasing prey up a slope tend to break off from an unsuccessful hunt earlier than they do on a level surface. While many in-game creatures are entirely fantastical, it doesn’t mean that even “unnatural selection” hasn’t left them less capable on sloping ground, although it does mean that a GM can surprise players by having an Underworld variation of a familiar beast have some mutation or other that helps them overcome a surface animal’s issues. For more options to make movement matter, reference the Momentum-rules.

Table 2-1: Diagonal Range and Movement Calculation – Moving or Firing Along a Slope
Move 5   10   15   20   25   30   35   40  
Slope angle Vertical distance Horizontal distance Vertical distance Horizontal distance Vertical distance Horizontal distance Vertical distance Horizontal distance Vertical distance Horizontal distance Vertical distance Horizontal distance Vertical distance Horizontal distance Vertical distance Horizontal distance
0 0.00 5.00 0.00 10.00 0.00 15.00 0.00 20.00 0.00 25.00 0.00 30.00 0.00 35.00 0.00 40.00
3.75 0.33 4.99 0.65 9.98 0.98 14.97 1.31 19.96 1.64 24.95 1.96 29.94 2.29 34.93 2.62 39.92
7.50 0.65 4.96 1.31 9.91 1.96 14.87 2.61 19.83 3.26 24.79 3.92 29.74 4.57 34.70 5.22 39.66
11.25 0.98 4.90 1.95 9.81 2.93 14.71 3.90 19.62 4.88 24.52 5.85 29.42 6.83 34.33 7.80 39.23
15.00 1.29 4.83 2.59 9.66 3.88 14.49 5.18 19.32 6.47 24.15 7.76 28.98 9.06 33.81 10.35 38.64
18.75 1.61 4.73 3.21 9.47 4.82 14.20 6.43 18.94 8.04 23.67 9.64 28.41 11.25 33.14 12.86 37.88
22.50 1.91 4.62 3.83 9.24 5.74 13.86 7.65 18.48 9.57 23.10 11.48 27.72 13.39 32.34 15.31 36.96
26.25 2.21 4.48 4.42 8.97 6.63 13.45 8.85 17.94 11.06 22.42 13.27 26.91 15.48 31.39 17.69 35.88
30.0 2.50 4.3 5.00 8.66 7.50 12.99 10.00 17.32 12.50 21.65 15.00 25.98 17.50 30.31 20.00 34.64
33.75 2.78 4.16 5.56 8.32 8.33 12.47 11.11 16.63 13.89 20.79 16.67 24.95 19.45 29.10 22.22 33.26
37.50 3.04 3.97 6.09 7.93 9.13 11.90 12.18 15.87 15.22 19.84 18.26 23.80 21.31 27.77 24.35 31.74
41.25 3.30 3.76 6.59 7.52 9.89 11.28 13.19 15.04 16.48 18.80 19.78 22.55 23.08 26.31 26.37 30.07
45.00 3.54 3.54 7.07 7.07 10.61 10.61 14.14 14.14 17.68 17.68 21.21 21.21 24.75 24.75 28.28 28.28
48.75 3.76 3.30 7.52 6.59 11.28 9.89 15.04 13.19 18.80 16.48 22.55 19.78 26.31 23.08 30.07 26.37
52.50 3.97 3.04 7.93 6.09 11.90 9.13 15.87 12.18 19.84 15.22 23.80 18.26 27.77 21.31 31.74 24.35
56.25 4.16 2.78 8.32 5.56 12.47 8.33 16.63 11.11 20.79 13.89 24.95 16.67 29.10 19.45 33.26 22.22
60.00 4.33 2.50 8.66 5.00 12.99 7.50 17.32 10.00 21.65 12.50 25.98 15.00 30.31 17.50 34.64 20.00
63.25 4.48 2.21 8.97 4.42 13.45 6.63 17.94 8.85 22.42 11.06 26.91 13.27 31.39 15.48 35.88 17.69
67.50 4.62 1.91 9.24 3.83 13.86 5.74 18.48 7.65 23.10 9.57 27.72 11.48 32.34 13.39 36.96 15.31
71.25 4.73 1.61 9.47 3.21 14.20 4.82 18.94 6.43 23.67 8.04 28.41 9.64 33.14 11.25 37.88 12.86
75.00 4.83 1.29 9.66 2.59 14.49 3.88 19.32 5.18 24.15 6.47 28.98 7.76 33.81 9.06 38.64 10.35
78.75 4.90 0.98 9.81 1.95 14.71 2.93 19.62 3.90 24.52 4.88 29.42 5.85 34.33 6.83 39.23 7.80
82.50 4.96 0.65 9.91 1.31 14.87 1.96 19.83 2.61 24.79 3.26 29.74 3.92 34.70 4.57 39.66 5.22
86.25 4.99 0.33 9.98 0.65 14.97 0.98 19.96 1.31 24.95 1.64 29.94 1.96 34.93 2.29 39.92 2.62
90.00 5.00 0.00 10.00 0.00 15.00 0.00 20.00 0.00 25.00 0.00 30.00 0.00 35.00 0.00 40.00 0.00
Table 2-2: Distance Covered When Falling at Various Terminal Velocities
Time in seconds Falling stomach down, limbs out Falling stomach down, limbs in Falling headfirst, limbs in
Feet (176/s) Meters (54/s) Feet (293/s) Meters (90/s) Feet (488/s) Meters (150/s)
1 16 5 16 5 16 5
2 64 20 64 20 64 20
3 143 44 143 44 143 44
4 255 79 255 79 255 79
5 398 123 398 123 398 123
6 574 177 574 177 574 177
9 1102 339 1291 397 1291 397
12 1630 501 2180 670 2295 706
15 2158 663 3059 940 3586 1103
18 2686 825 3938 1210 5056 1555
21 3214 987 4817 1480 6520 2005
24 3742 1149 5696 1750 7984 2455
27 4270 1311 6575 2020 9448 2905
30 4798 1473 7454 2290 10912 3355
33 5326 1635 8333 2560 12376 3805
36 5854 1797 9212 2830 13840 4255
39 6382 1959 10091 3100 15304 4705
42 6910 2121 10970 3370 16768 5155
45 7438 2283 11849 3640 18232 5605
48 7966 2445 12728 3910 19696 6055
51 8494 2607 13607 4180 21160 6505
54 9022 2769 14486 4450 22624 6955
57 9550 2931 15365 4720 24088 7405
60 10078 3093 16244 4990 25552 7855


A fall from a great height is one of the most common hazards facing an Underworld adventurer. At the end of a fall, a creature takes 1d6 bludgeoning damage for every 10 feet it fell, to a maximum of 20d6. The creature lands prone, unless it avoids taking damage from the fall.

In the real world, falling is every bit as dramatic as the gaming world, perhaps more so when you consider how quickly we cover distances when under the influence of gravity, even taking wind resistance into account. It takes roughly 3-and-a-half seconds to fall 200 feet, and the distance covered in six seconds (one round) is roughly 600 feet. This means that being able to cast a spell, for example feather fall, in order to save yourself is a vital ability! However, if a character is confident they can land safely, dropping into the endless depths is a quick way to travel vast distances. In Earth-like conditions with regard to air pressure and therefore resistance, it takes not much more than 30 seconds, or just five rounds, to fall a mile, while a kilometer takes approximately 21 seconds or 3-and-a-half rounds. This is because human-sized creatures have a maximum rate of fall (a terminal velocity) of close to 176 feet per second or 54 meters per second.

Beyond this speed, falling with limbs held against the body (293 f/s; 90 m/s) or headfirst (488 f/s; 150 m/s) offer different terminal velocities. When making an effort to overcome wind resistance in reduced air pressure, the fastest speed achieved by a plummeting human is 1,235 feet per second (380 m/s).

Interacting with Objects

A character’s interaction with objects in an environment is often simple to resolve in the game. The player tells the GM that their character is doing something, such as moving a lever, and the GM describes what, if anything, happens.

Characters can damage objects with their weapons and spells. Objects are immune to poison and psychic damage, but otherwise they can be affected by physical and magical attacks much like creatures can. Spells specifically note whether they affect objects. The GM determines an object’s Armor Class and hit points and might decide that certain objects have resistance or immunity to certain kinds of attacks (it is hard to cut a rope with a club, for example). Resilient objects may have a damage threshold.

Objects always fail Strength and Dexterity saving throws, and they are immune to effects that require other saves, unless such effects specifically affect them. When an object drops to 0 hit points, it breaks. A character can also attempt a Strength check to break an object. The GM sets the DC for any such check.

Food and Water

Characters who do not eat or drink suffer the effects of exhaustion. Exhaustion caused by lack of food or water cannot be removed until the character eats and drinks the full required amount.


A character needs one pound of food per day and can make food last longer by subsisting on half rations. Eating half a pound of food in a day counts as half a day without food. A character can go without food for a number of days equal to 3 + their Constitution modifier (minimum 1). At the end of each day beyond that limit, a character automatically suffers one level of exhaustion. A normal day of eating resets the count of days without food to zero.

To help further with the impact of gathering enough food to mount a successful long-term Underworld campaign, Foraging looks at foraging in various biomes while Hunting considers the effects of hunting in these areas.


A character needs one gallon of water per day, or two gallons per day if the weather is hot. A character that drinks only half that much water must succeed on a DC 15 Constitution saving throw or suffer one level of exhaustion at the end of the day. A character that only has access to less water automatically suffers one level of exhaustion at the end of the day. If the character already has one or more levels of exhaustion, the character takes two levels in either case.


A creature can hold its breath for a number of minutes equal to 1 + its Constitution modifier (minimum of 30 seconds).

When a creature runs out of breath or is choking, it can survive for a number of rounds equal to its Constitution modifier (minimum of 1 round). At the start of its next turn, it drops to 0 hit points and is dying, and it cannot regain hit points or be stabilized until it can breathe again.

For example, a creature with a Constitution of 14 (equal to a +2 Constitution modifier) can hold its breath for 3 minutes. If it starts suffocating, it has 2 rounds to reach air before it drops to 0 hit points.

Additional extended rules for breathing are presented in Breathless in the Underworld. These suggested guidelines are designed to render the exploration of deeper flooded caverns, traversal of longer corridors filled with deadly gases, or battles with creatures that attack with cloud-based breath weapons or spells, more thrilling. They also provide alternative methods and opportunities to rescue struggling allies and to experience alien environments with a greater degree of excitement than that provided by the basic rules.

Units of Stock

Using the units of stock rules streamlines the concerns of food, water, and comfort goods significantly while adding to the number of ways these concerns can impact the game in a meaningful manner.

Light and Vision

The most fundamental tasks of adventuring—noticing danger, finding hidden objects, hitting an enemy in combat, and targeting a spell, to name just a few—rely heavily on a character’s ability to see clearly. Darkness and other effects that obscure vision can prove to be a significant hindrance.

A given area might be lightly or heavily obscured. In a lightly obscured area, such as dim light, patchy fog, or moderate foliage, creatures have disadvantage on Wisdom (Perception) checks that rely on sight.

A heavily obscured area—such as darkness, opaque fog, or dense foliage—blocks vision entirely. A creature effectively suffers from the blinded condition when trying to see something in areas such as these.

The presence or absence of light in an environment creates three categories of illumination: bright light, dim light, and darkness.

Bright light lets most creatures see normally. Even gloomy days provide bright light, as do torches, lanterns, fires, and other sources of illumination within a specific radius.

Dim light, also called shadows, creates a lightly obscured area. An area of dim light is usually a boundary between a source of bright light, such as a torch, and surrounding darkness. The soft light of twilight, dusk and dawn also counts as dim light. A particularly brilliant full moon might bathe the land in dim light.

Darkness creates a heavily obscured area. Characters face darkness outdoors at night (even on most moonlit nights), within the confines of an unlit dungeon or a subterranean vault, or in an area of magical darkness.


A creature with blindsight can perceive its surroundings without relying on sight, within a specific radius. Creatures without eyes, such as oozes, and creatures with echolocation or heightened senses, such as bats and true dragons, have this sense.


Many creatures in fantasy gaming worlds, especially those that dwell underground, have darkvision. Within a specified range, a creature with darkvision can see in darkness as if the darkness were dim light, so areas of darkness are only lightly obscured as far as that creature is concerned. However, the creature cannot discern color in darkness, only shades of gray.


A creature with truesight can, out to a specific range: see in normal and magical darkness; see invisible creatures and objects; automatically detect visual illusions and succeed on saving throws against them; and perceive the original form of a shapechanger or a creature that is transformed by magic. Furthermore, the creature can see into the Ethereal Plane.

The Light & Darkness section considers the importance of light in more detail. It provides thoughts and guidance on how various pieces of uncommon adventuring gear can be used to provide unusual light as well as how naturally occurring light comes in many We also introduce Darkness as an entity.

Types & Anatomy of Caves

For all the wonders of the surface world, with its great mountains, steep valleys, sweeping plains, mystical jungles, and gleaming oceans touched by stern cliffs or curving golden sands, little which is above ground can properly prepare you for what the Underworld holds. With so much space to fill, the natural marvels that time produces induce awe and amazement to even the most experienced of delvers. Entire Upperworld civilizations rise and fall, with all the accompanying tumult and turmoil that entails, while the Underworld does little more than produce staggering natural architecture on a scale unimaginable to those living above it.

The below-ground environment you can create for your players could be forged by:

  • elements and forces both natural and extra-planar in nature
  • civilizations that are building out wards from within, upwards from below
  • magic, even the least of which can render unreachable places accessible
  • supernatural, prehistoric, and “mega” fauna variants
  • flora that, over the vast expanses of time, works its way into and along every available crack
  • intervention from deities, demons and devils
  • artisan humanoids whose intention is to harness all the wild energies the Underworld has for their own well-hidden purposes

Therefore, you can enhance and exaggerate the following examples from Earth to your heart’s content, knowing that the adventurers have access to resources that counterbalance the absence of technology. Remember, the center of the planet, much farther away than the sky, is your limit!


Structures below ground are referred to as subterranea, whether they are naturally occurring, such as caverns or volcanic pipes, or have come about artificially, for example catacombs or smugglers’ tunnels. The creation and shaping of natural locations over time or through sudden catastrophic occurrence is speleogenesis. Formed by water erosion, chemical processes, tectonic activity, microorganisms influencing regional chemistry, pressure both long term and/or sudden, and atmospheric influences, it is the first of these, erosion as a result of water movement—or hydrogeology—that produces the majority of standard-sized caves, sinkholes and tunnel systems, as well as the much larger caverns and canyons we find on our planet. The deepest known single vertical shaft in our world is over 1,900 feet/600 meters, with others close to this depth, while single passages up to 3 miles/4.8 km are known. The length of cave systems on Earth is a different matter, and total distances of over 100 miles/160 km are known to exist in many places, including underwater caves that are over 230 miles/370 km in length. It is thought that many of the current greatest systems are even longer as hidden exits and blocked linking tunnels are yet to be found or broken through. For example, as of 2019, two of the longest cave examples, the Mammoth (415 miles/668 km) and the Fisher Ridge (130 miles/210 km) cave systems in Kentucky are only 600 feet apart. This suggests that systems in excess of 600 miles/960 km are probably waiting to be discovered. This is where your campaign comes into its own.

There is every reason for fantasy cave systems to be much longer, and have deeper vertical shafts, more massive caverns, and probe much farther underground than anything on our own planet, and therefore send adventurers closer to more dangerous yet fantastic environs.

Now let us look in more detail at some natural examples.

Active Caves

These have water flowing through them at least some of the time. They can be inflow caves (via which a stream sinks or rain trickles into at the very least), outflow caves (from which a stream emerges at least some of the time) and through caves (traversed by a stream). Relict caves—in geology this is a structure or feature which has survived from a previous age—don’t have flowing water but may retain some from when the cave was more active or the water table was higher, although it will disappear eventually.

Solutional or karst caves are formed in soluble rocks including limestone, chalk, dolomite, gypsum, marble, and salt. They are dissolved by natural carbonic acid that comes into existence when rainfall mixes with organic compounds on or near the surface, and then seeps downwards along splits and ruptures. This acid rests in the original cracks and dissolves the rock so that they grow large enough over time to become caves, sinkholes, and underground drainage passages, and even large enough for exploring.

In addition, if the flow is coming from the ceiling or high up on the walls, formations such as flowstones, stalactites, stalagmites, helictites, soda straws, and full columns are brought about by calcium carbonate settling out of the gradual water movement. The most beautiful or memorable examples of these may attract intelligent creatures that view them as signs from their deity, turning the spot into an important center for their society.

If a karst is below the water table, it will eventually flood; in line with the above, if the water table drops, the karst then will eventually drain again. Both processes may take longer to complete than it does for the water table to move, especially if an area is prone to seasonal flooding or if there is little other activity to accelerate the process. It is possible for sulfuric acid to form caves because when hydrogen sulfide rises from the depths, it mixes with water that percolates to the surface, and then creates caves from below. This can take place with the acid in gaseous form, such is its strength.

Primary caves are formed at the same time as the surrounding rock, and this formation occurs over the passage of millions of years. However, from a gaming viewpoint, these actions become challenging hazards to deal with if magic induces them to take place during just a few rounds.

Examples of primary caves include: Lava tubes, where older, cooling flows create a thick crust that newer, hotter lava flows beneath, thereby possibly leaving a tube and caves. Lava caves include but are not limited to lava tubes, although caves aren’t likely to form without either a tube into or away from them, unless the flow in the tube cools sufficiently quickly to plug it before it travels into the cave. Lava tubes longer than 40 miles/65 km are present on Earth. Sea caves (or littoral caves) are a type of cave fashioned primarily by the wave action of the sea, actively forming along present coastlines and visible as relict sea caves on former coastlines. Erosion is ongoing anywhere that waves batter rocky coasts, but where sea cliffs contain zones of weakness, rock is removed at a greater rate along fault lines.

As the sea reaches into the fissures that are formed this way, they begin to widen and deepen due to the tremendous force exerted within the confined space, not only by direct action of the surf and any rock particles that it bears, but also by compression of air that begins to treat the supposedly solid rock as if it was something pliable.

Changes in sea level mean solutional caves can become sea caves and vice versa. On the surface this is perhaps more relevant where caves have formed at the edge of lakes large enough to have a tide, but in a gaming Underworld, changes to the great seas that exist deep below ground could easily produce this effect, not least if outflow points are blocked or if a vast quantity of the roof above a wide-but-not-deep sea falls into it and raises the level.

Corrosional or erosional caves are those that are carved entirely due to erosion by flowing streams that transport abrasive rocks and other sediments, as well as natural debris. Such is the pervasive nature of this process that they form in any type of rock, including hard rocks such as granite and tonalite.

However, for this to happen, there has to be some kind of inherent weak spot in the rock. This is needed to guide the water in a manner that concentrates its energy in tandem with the force of what is being carried along, and again means something like a fault line must be present.

A subtype of the erosional cave is the wind or aeolian cave, carved by wind-born sediments. These caves are often dry, and instead of column-forming stalactites and stalagmites often have sharp-edged boxwork and needle-pointed frostwork formations.

Many caves that were at first formed as a result of solutional processes are not unknown to undergo a later phase of erosion or enlargement by water that is above the level of permanent ground water.

Glacier caves are formed by the ice melting and the resulting water flowing along cracks within the glacier and channels between it and the surface it is standing on. The spaces created by the melted water are then further enlarged by the creeping of the ice, which, along with the pressure it produces, tends to shatter the caves and carry away the resulting rubble.

Although most glacier caves are initiated by water running through or under the glacier, this liquid usually starts on the glacier’s surface thanks to melting. This melt water then flows towards a hole or moulin (a cylindrical shaft that is almost always entirely vertical and can be many hundreds of meters deep) and exits at the glacier’s tip at base level. The water flow can then raise the temperature enough to encourage further melting, which in turn forms a cavity that fills with air (as ice takes up more space than water).

As the seasons progress, greater rainfall may refill the space with water, which exerts pressure on the glacier as it refreezes. This freeze-thaw action eventually can produce extremely large caves and significant tunnels from the surface to the base level exit points from the glacier. Once an air space has been created, air movement also helps with increasing the size of caves within the glacier by encouraging further melting during warm seasons and sublimation (solids becoming gases without the intermediate liquid phase) in winter.

Glacier caves are also formed by geothermal heat from volcanic vents or hot springs beneath the base of the ice. The resulting erosion can produce tunnels many kilometers long and chimneys hundreds of meters high, often highly unstable due to fluctuations in temperature.

Ice caves are similar to but different from glacier caves (sometimes the latter are mistaken for the former). The term is properly reserved for bedrock caves that: contain a large percentage of year-round ice; at least some part of which maintains a permanent temperature at or below 0o C/32o F; and water must have flowed into this cold area in order for the ice to form. Ice caves can contain cold traps (areas that hold onto cold dense air in the summer because warm light air can’t switch places with it), pockets of permafrost, and can possibly top up with winter snow that enters from thawing at any time of the year. Fracture caves are formed when regions of rock that are more likely to dissolve, including limestone and gypsum, are eroded or washed out from between the layers of rock that are more solid, such as granite. Eventually these “softer” rocks split before breaking apart and then fall away, finally leaving karst systems. Talus caves are created when large pieces of rock and other boulders fall from a mountain face as scree and end up in random piles that have access points into the resulting formations. These caves are never very large, nor are the overall karst systems extensive when compared to others in this list, not least because talus caves are extremely unstable in themselves.

Anchialine caves are almost always on or very near coasts as they are defined by holding mixed deposits of both fresh and salt water. While they can occur anywhere erosion is accompanied by the potential of the water types mixing, individual systems often contain animals and plants that are highly specialized to the particular environment, and as a result these karsts maintain unique biosystems that may be fragile, alien, unpredictable and difficult to prepare for. Coupled with these caves, anchialine pools are landlocked ponds and lakes that have a link to the ocean deep within them.

Physical Cave Formations

Within the various active caves types, there is a wide variety of formations that the systems can adopt as whole. An individual karst may well have more than one formation type within it due to several eroding influences, especially if it covers a vast distance underneath several surface environments, each of which encompasses different rock types and several erosive actions. This is especially true if a system travels deep underground, either trapping water or allowing it to disperse quickly, eliminating one type of erosion, but then being subjected to changes in heat or pressure instead. The six descriptions below are the most frequent examples of formation: Angular network systems are the result of erosion caused by chemical action on ever-increasing crevices in carbonate rocks. The passages produced are almost always high, narrow and straight, with pocket-like holes dotting the walls where pools of chemicals have collected. These systems are often closed off, which opens up the possibility of the party discovering ecosystems and societies that are completely unfamiliar with surface-world humanoids, in a manner similar to native South Americans first encountering Europeans with all the trials and tribulations that brought about.

  • Anastomotic systems are a series of narrow tunnels that erode separately but crisscross with each other slightly vertically and mostly horizontally. Junctions may form bigger chambers, while the greatest spaces are where all the individual passages eventually meet. These systems usually form along one layer of soft rock held against others of harder substances, and only meet up rarely with similar systems thanks to concerted vertical erosion.
  • Pit systems are simply vertical shafts and sinkholes rather than having their own horizontal karsts. Many pits become flooded, with the water level in them dependent on the surrounding rock. If this is not porous, it is possible for the surface to be above the water table if liquid can’t flow away quickly enough. Pits may also have waterfalls flowing down them, hampering their use.
  • Branchwork systems are similar in appearance to the way streams flow down hillsides in that the tunnels and caves are initially small, but slowly meet up. This allows greater erosion to take place and form larger caverns and passageways. They are usually formed as a result of a number of sinkholes letting a small amount of water in on an occasional basis, rather than a single constant flow producing erosion at all times.
  • Ramiform systems are another karst type created chemically, in this case thanks to hydrogen sulfide mixing with the water responsible for the erosion. They occur where there is a moving water table; this means the spaces can be very large, are almost always irregular, and consist of caverns, passages and galleries in unexpected configurations, for examples small caves high up on the walls of a larger space.
  • Spongework systems are karsts that come about as a result of spaces created by one erosion type becoming randomly and irregularly joined together thanks to a different one. The waters that do this usually have varying chemical components that make spaces and tunnels in different way and forms, so that the final karst can’t readily be identified as one type or another, yet looks similar to the inside of a sponge in overall nature.

Living Caves

All of the above are considered living caves if there is any amount of water flowing through them, whether this is a constant river or even the slightest seepage. So long as erosion and/or creation of features takes place, the space is “alive” for spelunking purposes, and it will continue to grow if erosion outstrips creation, or may even refill thanks to the development of draperies, flowstones and columns at a faster rate than the water or other activity can wear them away, or end up as a gallery decorated with the most wonderful natural sculptures that attract Underworld humanoids from far and wide.

Unused living caves are often clean, dust-free, and may reflect light when it is taken into the space. All this changes once they start to be used; if a space lacks one or more of these three elements while water is present, it is probably being lived in currently, or at least has been occupied recently. Alternatively, if mud is present, it is quite likely the cave does not have anything in it unless the creature is specifically a mud-dweller.

Dead Caves

The opposite of a living cave is a dead cave; the differences between the two are clear and obvious. Water no longer flows through a dead cave, although there may be a lot of evidence of historical activity, and erosion almost certainly isn’t taking place (in a game world where Underworld air currents are more prevalent, it may be that particle erosion is still occurring).

Many rocks need humidity to stay at their strongest, and dead caves become dry and, in some instances, quite fragile, meaning it is much more likely that rubble is strewn around. They do not sparkle when light is brought into them, and they are almost certainly silent; without water, a nonmagical ecosystem cannot be established. In game, this may mean they become the domain of purely elemental creatures, ones able to live without water and having repurposed the caves from their original use now creatures of flesh and blood have moved on. They can also be the home of ancient undead, although those with skin may be brittle and the color of parchment, while those of bone could have become as hard as the rock that surrounds them yet be missing finer, smaller bones. Such venerable sentinels may signal the historic existence of some antediluvian society.

Some dead caves do contain liquid, but it is likely to be an ancient pool of standing water. Despite the probable age of such a pool, the contents are almost certainly potable thanks to the absence of sun and an unchanging cold temperature. Microbial threats may still exist in the water, though.

In game, dead caves can be used as an indication that the party is going the wrong way or is unlikely to encounter any further activity, or they can be used as a demarcation line to more hostile terrains or a belligerent realm that needs to be traversed to discover new regions.

The Underworld as a Whole

An Underworld containing extensive tunnels and caverns as a result of erosion is considered viable in three main theories: the Hollow Earth theory, the Isolated Pockets theory, and the Swiss Cheese theory. Finally, there is an overarching compromise, the Partial Connection theory. It is worth bearing in mind that some elements of these theories are completely fantastical as they came about during times when the workings of the planet weren’t understood.

Hollow Earth Theory

This theory has a number of variations, most of which stem from a similar idea which appeared in many ancient myths and religions that suggests there are caves, caverns, sinkholes and other entrances that lead directly to the Underworld, a place of origin or afterlife. Many of these talk about cities, gardens, or sites containing strange flora and fauna, as well as the existence of either supernatural ancestral beings capable of great feats or the dead in their afterlife existence.

The structural ideas emerging from these folklore tales suggest that Earth consists of a hollow shell many hundreds of miles thick, usually with large entrances at either the North Pole, South Pole, or both, and that these sinkholes drop great distances into the planet reaching entirely through whatever crust is believed to exist, but then ideas diverge, ranging from:

  • the interior being almost constantly only dimly lit, with creatures either flying or levitating, or being able to walk on the inner surface.
  • the interior containing an inner sun a few hundred miles across that works in the same way as the celestial sun. An additional variation on this idea is that there are two suns.
  • the interior having a varying number of concentric inner shells, possibly with an inner core. Each inner sphere has its own atmosphere, magnetic polarity, and direction of spin, and is capable of supporting ecosystems that are different from those on the surface world.
  • the interior is simply a variation of the exterior, but set sometime in the past, usually prehistoric, and can be interacted with by surface dwellers equipped to travel into the depths. This Underworld has its own light, but this does not affect the surface world in any way.
  • the interior is the repository of ancient machines capable of time-and-space flight, and that it is the resting place of ancient astronauts, flying saucers, and other equipment capable of traveling the multiverse (and other planes, in game).

Isolated Pockets Theory

This theory suggests that erosion hasn’t linked the various caverns and tunnels it has produced and that instead of forming a vast, planetwide web of passages it has created thousands of individual, colossal, karsts. It does mean that there may be entrances to these individual systems, notwithstanding that pressure or cave-ins will have closed many off, but if erosion has taken place, then at some point there was a way in, even if it was for water or chemicals.

At present, this seems the most likely condition of the Underworld on Earth, with the evidence of cave systems beneath every continent as well as the sea bed that only stretch hundreds rather than thousands of miles, but is perhaps least helpful from an in-game viewpoint.

In addition, this theory also allows for an assortment of unrelated cultures and realms to develop, but doesn’t deal well with the idea of races who dislike the surface occurring everywhere within the Underworld, unless either magic, in particular planar travel, is employed, or these races went against their fears and principles and crossed over the surface at some point in their history.

While adventurers could use this same magic or planar travel to overcome any errors made in choosing which isolated path to follow, many lower-level parties will have to face up to the frustration of returning to the surface and using another entrance if you decide your Underworld con forms to this theory.

Swiss Cheese Theory

As an opposite of the Isolated Pockets theory, the Swiss Cheese theory believes that no great spaces or passages exist in the Underworld, particularly not at the core, but instead that every one of the uncountable smaller caves and tunnels links to another, and that any one point within the Underworld can be reached from any other just by knowing where you are going.

This idea supports the appearance of in-game races at many places within the Underworld, and of common languages that might be spoken, or at least the many components that are featured wherever you travel so that variations can be described simply as regional differences in dialect.

However, this theory would suggest that each Underworld race is familiar with all the others, which is not the case. While it may be true that, say, orcs and goblins have found ways to move around near surface level, it is almost certainly also true that fewer occasions for erosion have prevented navigable routes from being properly created, in that they are too small or that the hazards are too great, and that the increased pressure and constant movement of the greatest depths simply closes these paths soon after they come into existence.

Partial Connection Theory

This theory is the best of all worlds, in that it accepts the idea of all Underworld regions sharing a connection, but not that the routes are straightforward either to find or to follow. It acknowledges that any links between regions are almost certainly dangerous beyond experience, are wracked by the greatest of hazards, and are both frequently and regularly impenetrable as the erosion that opens the space also creates passage-blocking rubble, while pressure and movement reseal any spaces that do come about.

In game, the difficulties posed by this theory require the party to be fully prepared for travel through elemental conditions, particularly whole regions flooded by water, lava, and crushing earth, or find them devoid of suitable air due to lack of currents.

When the Underworld is Above Us

As a final point for consideration, neither of the two deepest karst systems currently known on Earth—the Veryovekina Cave (7,257 feet/2,212 m deep; 8.43 miles/13.50 km long) and the Krubera-Voronya Cave (7,208 feet/2,217m deep; 8.35 miles/13.43 km long)—get to reach below sea level as they both begin a greater distance above the “zero feet” surface than they drop down. It is worth reading further about both systems.

More Rewarding Climbing

Here’s to a life as long as your rope, a heart as stout as your rope, adventures as many-stranded as your rope, and allies as unfailing as your rope! —

Toast of the Stoneholme Spelunkers’ Society

Climbing is a pretty straightforward affair—you usually employ Strength (Athletics) to scale an obstacle, and it costs you half your movement, mitigated by whether you have the correct feat or class ability, such as the rogue’s Second-Story Work feature. Of course, the climber’s kit does exist, but as written it only prevents you from falling more than 25 feet from where you used it to anchor yourself, plus it weighs a whopping 12 lbs. In our world, climbers are obsessive about weight, removing the carton from rolls of toilet paper for example, so even the stated 12 lbs. can seem a lot, particularly in a world where magic exists. Consider allowing the following variants of climber’s kits:

  • Elemental Climber’s Kit (Air): This climber’s kit has been infused with elemental air, making it weightless.
  • Elemental Climber’s Kit (Earth): This climber’s kit has been infused with elemental earth. The elemental energies leave hand- and foot-holds for those following a character climbing with it. The footholds last for an hour before seamlessly reintegrating into the wall. The kit can only create hand- and foot-holds in earth or stone, not in other materials. It has 10 uses.
  • Elemental Climber’s Kit (Fire): This climber’s kit has been infused with elemental fire, leaving a clearly visible trail of ephemeral wisps of harmless embers behind the climber. This allows for easy and safe use of the Planned Route action by those following behind, as it clearly illuminates the path, regardless of distance. This kit can be activated or deactivated as an action and has 10 uses. Upon deactivation, the whole illuminated trail winks out. Reactivating the kit does not restore the previous trail, and instead starts leaving a new one.
  • Elemental Climber’s Kit (Water): This climber’s kit has been infused with elemental water, making mystic use of surface tension. This kit can be activated or deactivated as an action and has 10 uses. An activation lasts for 1 hour. After activation, the kit negates increases to the climbing DC or disadvantage on Strength (Athletics) checks made to climb due to surfaces made slippery by liquid. Additionally, it lets you climb vertical stretches of water, such as waterfalls, as though they were made of stone. Fungal Spore Climber’s Kit: This climber’s kit contains pouches of brightly fluorescent, sticky spores that adhere to rock, earth, and similar surfaces, leaving a clearly visible trail of ghostly illumination behind the climber. This allows for easy and safe use of the Planned Route action by those left behind, as it openly illuminates the path up to a distance of 500 feet. This kit can be activated or deactivated as an action by opening or closing the spore pouches. The spore pouches are sealed airtight, and upon opening a pouch, the spores lose their glow within one hour. Each pouch contains enough spores to cover a distance of up to 500 feet.
  • Masterful Climber’s Kit: This climber’s kit is made of superior materials, making it lighter.
  • Moss Climber’s Kit: This climber’s kit uses elastic ahooling moss ropes, and the climber may adjust the stretch they can fall when securing themselves. The climber can choose to adjust it between 10 feet and half the maximum length of the moss rope. The downside of moss ropes is that they are highly flammable—they are vulnerable to fire damage.
Table 3-1: Exotic Climber’s Kits
Name Price Weight
Elemental Climber’s Kit (Air) 5,000 gp
Elemental Climber’s Kit (Earth) 3,500 gp 12 lbs.
Elemental Climber’s Kit (Fire) 5,000 gp 6 lbs.
Elemental Climber’s Kit (Water) 1,500 gp 12 lbs.
Fungal Spore Climber’s Kit 50 gp 12 lbs.
Masterful Climber’s Kit 50 gp 6 lbs.
Moss Climber’s Kit 100 gp 6 lbs.

Now we have looked at the tools for the job, let us discuss some challenges of climbing within the context of the Underworld. The default rules presented by 5E work perfectly well to simulate the usual exploration of dungeons, cities, and so on, but the Underworld is an inherently three-dimensional environment, and verticality is half the fun of exploring these wondrous vistas. To offer a sense of consistency, Table 3-2 gives sample DCs for climbing.

Table 3-2: Climbing by Surface
Surface Climber’s Kit required? + Sample DCs with a Kit * Sample DCs without a Kit *
Rough, natural foot/handholds No 5 5
Normal, natural foot/handholds No 10 10
Smooth, natural foot/handholds No 15 15
Rough, no foot/handholds Optional 10 15
Normal, no foot/handholds Optional 15 20
Smooth, no foot/handholds Optional ** 20 ** 25 **
Overhang up to 45°, rough Optional 15 20
Overhang up to 45°, normal Optional + 20 25
Overhang up to 45°, smooth Optional + ** 25 30
Ceiling, rough Optional + 25 30
Ceiling, normal Yes+ 30
Ceiling, smooth Yes+** 30

+ Depending on the circumstance, a GM might decide that a surface simply requires a climber’s kit to scale. If the character has a climbing speed, this allows them to scale such obstacles as well. If you want to retain a DC for characters with climbing speeds, detract 15 from the suggested DC.

* It is suggested to grant advantage on these checks to characters that have a feat or feature that allows them to climb at their full Speed.

** When scaling smooth walls without foot/handholds, more realistic games may consider this to be impossible instead. As written, the DCs for these entries represent brief bursts of wall-running/scrambling and should be considered more suitable for high fantasy games.

It must be remembered that a predominantly 3-D environment presents challenges for exploration; not every character class is suited for prolonged climbing. If you are a burly fighter or barbarian, you may be able to scale some massive canyon, but neither your wizard nor warlock companion is likely to have your prodigious Strength. Usually, the core rules handle this with utility spells such as spider climb or fly that allow spellcasters to bypass obstacles.

However, while exploring the realms below, far beyond the trappings of civilization, any spell that may mean the difference between life and death has the issue of duration and scale to consider; spider climb only lasts up to an hour, which may not suffice to reach the top of that vast chasm, while fly can only be maintained for 10 minutes. Both spells require concentration, making them risky. In real life, preparation and planning are crucial components for climbing, and the following allows less brawny characters to carry their own weight without taking away from their fellows.

Planned Route. You can take 1 minute to observe an intended path of area of up to 60 feet. You plan the best possible route to climb the observed area. You have to be able to clearly perceive the entire area in order to engage in the planning of a route—usually, this means using eyes to see the area, but vestraadi with their sonar in particular, are exceedingly good at planning routes. The total number of plans of such stretches of distance you can memorize at one time is equal to your proficiency bonus + your choice of either your Intelligence or Wisdom modifier. Explaining the plan to scale a stretch of distance takes 1 minute and grants anyone attempting the climb advantage on checks to scale the stretch, and reduces the Strength (Athletics) DC to climb the stretch by up to 10, to a minimum of DC 10.

Creating Hand- or Foot-holds. You can take thrice as long to climb a surface, but leave hand-or footholds, using pitons and similar equipment for your less athletic comrades. Doing so reduces the DC to climb the surface by 5, to a minimum of DC 5. A Planned Route can thus be reduced to DC 5. Note that the act of making hand- and footholds is exceedingly noisy and leaves obvious traces, thus making an enemy’s tracking of escaping targets all too easy.

To roll or not to roll? The two actions presented here do not have DCs per se, because Planned Route is intended to provide an option for the party. Additionally, the action requires the party to split (e.g. if the rogue climbs ahead to plan the route) or the expenditure of resources like fly to plan stretches with a lot of overhangs. Furthermore, the use of light and the requirement of sight can become additional complications.

It should be noted that gaining the benefits of a Planned Route is not always possible—a perfectly smooth wall remains so, and no amount of planning will change that without considerable additional effort. If the GM wants to simulate a particularly dangerous or hard-to-plan climb, they can use the Suggested DCs column of the Table 3-2: Climbing by Surface as a guideline.

Some surfaces may be harder to plan than climb, or vice versa. Requiring a check to plan a route should be the exception though—checking every 5 minutes bogs down the game. Similarly, creating hand- or footholds can be particularly helpful, but the noise and time required are sufficiently detrimental to not invalidate instances where using spells such as spider climb and similar options make more sense. In short, the rules above are here to make the climbing experience more exciting and streamlined for the entire party.

Finally, do bear in mind that resting while climbing a sheer surface will require ingenuity and possibly special equipment, and that exhaustion is a factor for prolonged climbs on any type of surface, even those that are stairway-like if they extend many hundreds of feet.

While there are many reasons for a dwarf to delve, the most obvious is the desire to create a passageway in the quest for veins of valuable ore, beds of precious gems, or stockpiles of hard, hot-burning coals. Initial delving in these cases is usually undertaken in an exploratory attempt to locate the commodity being sought. Experienced miners tell from the types of rock, the location of strata, and other clues, where this underground wealth might be set.

In addition, specific clues, such as gold dust and even nuggets being carried downstream by f lowing water, can give an indication where to dig. Inexperienced miners must gain this knowledge the hard way, by trial and failure and trying again. Fortunately, I had some luck, and I knew how to persevere.

Such initial exploratory tunnels are generally straight, and relatively narrow. In many cases, where the rock is porous or soft, shoring timbers will be needed to protect against collapse. If the tunnel slants steeply down (or up) steps may need to be carved into the floor. Vertical tunnels are sometimes necessary, and these of course require ladders, spiraling stairways carved into the walls (or built from wood), or mobile platforms that can be raised or lowered, often by means of a winch.

That reminds me; have I told you about the ingenious machine I invented? It’s quite a clever contraption, and by now is used in probably half the mines of all dwarfdom. It uses a series of pulleys, and the rope or chain wraps back and forth around the pulleys so that it can pull three or four times the weight allowed by a simple single attachment.

This means that when it is attached to a crane, or anchored at the top of a shaft, it can be powered by beasts of burden, waterwheels, or the sturdy strength of a team of operators, of which I’ve found dwarves to be the best suited. I understand some gnomes are even working on a device that would let the thing be powered by steam! Although that sounds far too much like it relies on magic, to me. Regarding the “machine” that claims to have invented, the astute reader has no doubt realized that he is describing a simple block and tackle, which can indeed dramatically improve the mechanical advantage provided by a single pulley. Of course, the block and tackle has been understood and utilized by dwarves, gnomes, humans, elves, and most other intelligent creatures for a period of time going back countless generations.

Truth be told, anyone entering an area where there is evidence of a dwarf mining but the nearest settlement is deserted may want to be additionally cautious if natural pillars have been replaced, because it is quite likely the area has been dug out beyond safe limits.

Once the presence of the mineral or stone within the area of the mine is confirmed, the miner will follow the natural contours of the deposit. The excavated tunnels become much more irregular during the process, angling this way and that, often diverging into two or more different directions. The width of the delvings can also vary extensively, as sometimes even large chambers must be created to gain access to every bit of the mineral wealth discovered there. Leaving no stone unturned has never been more important than to a miner.

Just as long tunnels must often be shored up with timber or stone supports, so too must the ceiling of a large cavern be propped up with pillars to protect against a cave-in. The most effective way to do this, of course, is to simply leave intact columns of the natural rock at intervals close enough to hold up the roof overhead. In cases where the deposit is so dense that it becomes profitable to chisel through these potential pillars, they must be replaced with timber, iron, or stone support shafts constructed by the delver specifically for that purpose.

Once the presence of the mineral or stone within the area of the mine is confirmed, the miner will follow the natural contours of the deposit. The excavated tunnels become much more irregular during the process, angling this way and that, often diverging into two or more different directions. The width of the delvings can also vary extensively, as sometimes even large chambers must be created to gain access to every bit of the mineral wealth discovered there. Leaving no stone unturned has never been more important than to a miner.

Just as long tunnels must often be shored up with timber or stone supports, so too must the ceiling of a large cavern be propped up with pillars to protect against a cave-in. The most effective way to do this, of course, is to simply leave intact columns of the natural rock at intervals close enough to hold up the roof overhead. In cases where the deposit is so dense that it becomes profitable to chisel through these potential pillars, they must be replaced with timber, iron, or stone support shafts constructed by the delver specifically for that purpose.

Of course, mining is not the only reason a dwarf will delve through solid rock. Sometimes you just want to make a shorter passage between two locations, so you bore a tunnel from here to there. Or you build a bridge to span a chasm, or drill a shaft to allow travel deeper into the ground, or up toward the surface. The principles of mining remain, but a team of dwarven delvers, well equipped with tools and fortified with a reliable supply of ale, can create an underground route to just about anywhere a traveler would want to go.

An Overview of Excavated Spaces

The list of excavated spaces that lead beneath the surface and from there allow access into the Underworld is long and varied. Almost all of them will be familiar to players and adventurers alike, and don’t really need much detail, as the differences between them are often more to do with origin and story rather than significant attributes. Therefore we provide just a few examples here to encourage imaginative use of them:

  • Basements including: arsenals, cellars, furnace rooms, silos, store rooms, substructures, underbuildings
  • Burrows including: dens, hideaways, lairs, refuges, retreats, root balls
  • Crypts including: catacombs, grottoes, mausoleums, necropolises, ossuaries/astudans, sepulchers, vaults
  • Dungeons including: angstloch holes, black holes, bottle dungeons, oubliettes, subterranes, underground cells
  • Mines including: boreholes, cave-ins, pits, quarries, sinkages, sinkholes, tube wells

However, it is worth looking at the five broad categories with greater scrutiny as they all have a distinct purpose, and therefore offer different reasons for entering them and using them as ways into the depths.

Basements and their like are almost always used as a utility space or for storage of some type, and as such will have security when you try and enter them from above, but often little thought to what might happen from below. The same factors that affect the building above them—climate, surrounding material, likelihood of earthquakes or flooding, cost to build—affect the basement.

Burrows and similar excavations are invariably created by beasts, although some humanoids may resort to them in difficult circumstances. They almost always start as straightforward shelters against both predators and the weather, before developing into a more permanent habitat, and finally move on to somewhere that has access to the Underworld, often by chance rather than design. The opportunity for an accidental passage to deeper tunnels to occur—or for the chance to explore and find or create an entrance naturally—increases with the length of the burrow.

In the real world, rabbits and similar vertebrates’ networks of chambers and tunnels can stretch for lengths of thousands of meters and cover a variety of levels. Invertebrates also create burrows; real-world structures can be impressive when you consider the size of the builders, so scaling up to in-game monstrosities such as ankhegs, remorhazes, and purple worms will produce systems that provide plenty of opportunity for spelunking. The stability of vertebrate-created burrows usually comes about because they are “fit for purpose”, and habitation by the makers keeps the side rounded and compact. Invertebrates often soak the walls of their work with mucus, which serves to seal the tunnels for security at the same time as preventing water breeching their home. Adventurers will find all manner of beings in burrows besides the primary active hosts, as often they are filled passively by secondary dwellers who adopt at least parts of the system as their own.

Crypts and other below-ground chambers of this ilk are usually made of stone to provide protection for their contents, which almost always are coffins, sarcophagi, bodies (whole or parts) and other relics of a religious nature. In most holy buildings, there would be a specific position for the main crypt, but over time, others would be built, perhaps by individuals or families, or in order to store the bodily remains from a battle that saved the exact site or settlement that houses the hallowed building as a whole.

Another purpose for these sacred sites is to have somewhere secure and sanctified to conduct rites. Relics often became the focus of detailed ceremonies, the nature of which was hidden from non-believers or those who were critical of the religion. In Late Latin, “crypta” not only meant “vault”, but also “hidden” in certain circumstances, and many such spaces were clearly built to keep these ritualistic items and actions secret.

Dungeons and other spaces of that nature have not always been dark, dank subterranean cells we now think of, but invariably have been associated with holding people securely and by some measure, safely. Such prisons were initially in the strongest section of a keep, in spots that were carefully guarded and less accessible than elsewhere, but as time went by, they gained a reputation of being places to fear thanks to the idea of torture or “forgotten incarceration” playing major parts in their reasons for being built.

Oubliettes and bottle dungeons are the next step of this fear-inducing process, as they are built and shaped unlike standard rooms in prisons, with even access being in an unusual place such as a hatch or hole in the ceiling used as ingress. “Angstloch” holes are an example of where fearfulness served a distinct purpose; the original name for them may have been “angustus loch”, or “narrow hole”, but the German words “Angst” and “Loch” changed the meaning to “fear hole”. Other basic uses for dungeons that can be employed within an adventure include:

  • holding too many people in too small a space, leaving them open to disease and malnutrition
  • holding the innocent so the gaoler can instill a sense of superiority over them
  • leaving people in places where supporters simply cannot get to them
  • holding political and religious rivals in order to keep them out of the way.


Mines and mining in the game serve the same purpose as in the real world; they are there to uncover and remove ores and minerals for various purposes, although there will almost always be an economic element to the digging. While they usually comprise straightforward tunnels and shafts, their aim is always to find a way to the next lode, seam or vein of ore. As such, they may be reasonably straight at the entrance and initial route from the surface, but they can quickly become less regular and organized as they follow the deposits of metal or gems, or substances such as coal and salt. When the party comes across a previously-dug mine it will either be active, in which case it will be protected and there will almost certainly be someone working at all times, or inactive, after which it may well have become the lair of a beast, the home of humanoids, or the resting place of undead, depending on the reason for it being abandoned. Whilst mines are usually worked until it is no longer economically viable to do so, they may be forsaken for other reasons such as war, a curse (perceived or actual), too many accidents, another chance to mine with less effort, or something rising from below to chase the miners away.

Generally, the way mines operate can be split into two categories: surface mining and underground mining. Each of these can again be split into two categories: placer mines and lode or seam mines. Placer mines are deposits of ore or minerals within riverbeds, beaches and banks, or other alluvium material, while lode mines are where seams, layers or granular distributions of the material are found and developed. Both types of mine occur underground, but the latter, lode mines, are usually much more lucrative and occur much more frequently.

The denizens of the Underworld are usually more interested in the water of a stream than the possibility of tiny quantities of ore or slivers of gems, but that does not mean placer mines don’t exist. Because they involve carefully filtering materials like sand, dirt, mud and shale from flowing water before combing through the collected substances to extract the ore or gems from within it, it takes equipment designed to allow smaller and smaller particles to fall through increasingly finer meshes, and enormous patience. The process eliminates large chunks of waste initially, and then work steadily towards leaving the desired items behind as the finest grains of detritus are removed.

Experts are able to spot the finest of motes of gem or metal, and work sites considered barren by novices. They are also more adept at spotting how a water flow has passed a lode and eroded the rock around the valuable content further upstream. In addition, they appreciate how changes to the course allow ore and gems to drop as the water slows temporarily. A canny placer miner employs different techniques at various points along a river, including creating artificial still pools and adjusting the water’s direction of travel.

Creating mines of any type requires much time and effort, as well as being a venture fraught with danger and expense. As others become aware of the miner’s plans and actions, it is likely that bandits will try to take it from the claimant, tax collectors will want the regent’s share handed over, indigenous people will want to protect sacred and traditional sites, or beasts may find the excavation a desirable lair.

With the Underworld being active and vibrant in three dimensions, it may be that other workers are digging from below in order to reach whatever wealth is there to be mined, and a dispute begins as to who has the right to excavate at that strike. Initially digging out tunnels is hard work and takes up a lot of energy, while maintaining them is less difficult but demands different skills. Like lifting and carrying, an adventurer’s Strength score determines the amount they can excavate in assorted types of ground in cubic feet. How Much Can You Mine? To determine the amount of cubic feet that a character can mine in a given 8-hour shift, multiply the character’s Strength modifier with the multiplier from Table 4-1, and round down the result. A character can only mine for 8 hours on a given day before they need to finish a long rest to keep performing at full efficiency. A character can keep mining for longer than 8 hours by succeeding on a DC 10 Constitution saving throw to avoid gaining a level of exhaustion. The type of ground can increase the DC as well: Soft ground increases the DC by +2, Hard ground by +5, and Very Hard ground by +10. Additionally, for every 8 hours beyond the first 8-hour interval spent mining, the DC increases by a further +2. Subject to the GM’s discretion, eating double rations or consuming twice as many units of stock provides advantage on the saving throw.

An exhausted character halves the amount of cubic feet they mine in an 8-hour period. Humanoids other than those listed below should use the values in Table 4-1 as a guide.

To recap, if a character mines for longer than 8 hours on a given day, and then gains a level of exhaustion, they halve the progress made while they are exhausted, as stated on Table 4-1.

In addition, only a set maximum number of each race of miners can work in a 10-foot-wide tunnel, with at least half the number being experienced at using reach tools, either via their background, magics, or appropriate tool proficiency. This value also assumes that some of the working party are removing debris.

When a vein or seam is first discovered, its direction of run must be established. Then, as mentioned, once it is being mined it will change direction periodically, making it more difficult to trace. Successfully keeping track of it requires a 10-foot-wide by 10-foot-high by 10-foot-deep tunnel or shaft to be cleared and then an appropriate skill used to spot the ore or gems and further develop the claim.

To picture this, imagine a large 30-foot-by-30-foot-by-30-foot cubic chamber (a huge Rubik’s Cube, for example) which is formed of 27 smaller 10-foot-by-10-foot-by-10-foot cubes; the miner digs through the middle of one face and finds the seam or vein in the center of the cube. From here, in theory, the seam has 27 options: it can either not go anywhere, or travel to 26 other cubes. However, one of these is back toward the previously-dug cube, which is impossible and so leaves us with 25 directions of run.

Table 4-1: Mining Rates
Race of Miner Strength Score Multiplier for Types of Rock Maximum number of Miners per 10-foot-wide tunnel
Very Soft Soft Hard Very Hard
Gnoll, Halfling, Human 4 3 1.5 0.5 Gnoll 8, Halfling 16, Human 12
Gnome, Kobold 4.5 3.5 2 1 Gnome 16, Kobold 16
Goblin, Orc 5 4 2.5 1.5 Goblin 16, Orc 12
Dwarf, Hobgoblin 5.5 4.5 3 2 Dwarf 16, Hobgoblin 12
Ogre 8 6 4 3 6
Hill Giant 12 10 7 5 4
Fire Giant, Frost Giant 14 12 9 6 Both 4
Stone Giant 20 16 12 8 4

In order to reach these other cubes, the seam runs in three dimensions. For example: With these three directions in mind, we can see that if a miner has been working forward along a straight, horizontal seam, in relation to the central space holding the ore or gems, they have come from “straight, level, backward”, i.e. from the already-cleared 10-by-10-by-10-foot cube. This is the “impossible” direction in this case, and either the roll can be repeated, or the GM decides secretly where the seam runs. If the result “straight, level, sideways” comes up, the seam or vein only fills the volume of rock it was found in, and then does not run anywhere further. There is a roughly 1-in-20 chance a seam peters out immediately, and a 1-in-30 chance it goes in the “impossible” direction back to where the miner has already mined. If this occurs, either reroll the direction of run or consider the seam as spent.

With their character looking straight at the find of ore or gems, a player rolls 3d10 and consults the following table to find the initial direction of run.

Then, after two additional 10-foot-by-10-foot-by-10-foot sections of rock are cleared, the GM does the same thing, but secretly, to establish which path it now runs along as it gradually bends back and forth in those three dimensions.

A vein will not always continue, of course, whatever the direction of run may suggest. It could end at a different type of rock that is much harder and stopped the production of the metal or gems. It could open out onto a cliff face, either one that has been a gap for millennia, in which case it is unlikely to carry on the other side of the chasm even if it could be picked up, or perhaps the rift was cause by a ceiling collapsing far below, taking a slice of weaker rock with it. In this case, the GM can use the direction of run table Table 4-2: Seam Direction of Run to trace the path of the seam across the void and allow it to continue on the other side.

d10 1st Direction of Travel 2nd Direction of Travel 3rd Direction of travel
1 Left Up Forward
2 Left Up Forward
3 Left Up Forward
4 Straight Level Forward
5 Straight Level Forward
6 Straight Level Sideways
7 Straight Level Sideways
8 Right Down Sideways
9 Right Down Backward
10 Right Down Backward

More dangerously, the direction of run could end up opening onto a lake, or a lava tube, or similar, releasing the resultant materials back into the mine, rendering it unworkable. It could run across the ceiling of a vast cavern, and the miner only finds this out when the path collapses and drops them hundreds of feet to the floor below. Or it could run beneath a section of unstable rock, so that once the worked tunnel reaches a certain length, the ceiling collapses and refills all that has been cleared, potentially trapping the miners in a living tomb! In addition, a “room and pillar” tunnel or dig must be protected from the pressure of the rock above it by shoring it up. Wooden, stone, or in the case of many Underworld societies, hardened mushroom props should be placed at suitably frequent points. For a 10-foot-square section of tunnel, a roof brace should be at least 10 feet long and 1-foot square, while wall supports are 9 feet long (to reach from under the roof brace to the floor) and also 1-foot square.

This bracing may also be required to maintain shafts, where diagonal positioning is essential or sheets of suitable material are secured in place. Collapsing walls will block the tunnels beneath and require further excavation at best, and may once again trap or kill miners at worst.

The material these supports are made from dictates their weight, and therefore how many miners it takes to bring them to the excavated section and successfully position them. When digging through more solid rock types, it may be possible to remove the rock in an appropriate size and shape to make braces in situ, but it takes great skill and slows the excavation down considerably.

Of course, if ogres or giants are excavating the claim, the tunnels probably will be larger, and bracing will have to be both longer and thicker in order to do the job. As the width or height of a tunnel doubles, the associated prop has to at least double in all three dimensions. For example, if a tunnel increases from 10 feet to 20 feet height, the side supports go from 1-foot-by-1-foot-by-9-feet in size to 2-feet-by-2-feet-by-18-feet.

An aspect of “room and pillar” excavation undertaken by the most experienced miners involves extracting the seam as far as it goes while placing props that barely do their job. When the ore or minerals are exhausted, but it is known that the claim goes upward, the miners undertake “retreat mining”; they remove the props and allow the ceiling to cave in deliberately, hopefully revealing more reward for their effort in the already-broken up rock. While the dangers of this method are clear, the time saved by having the rock partially fragmented is considered worth the risk.

The Rewards of Mining

Metals fall into five categories in relation to abundance: very common; common; uncommon; rare; very rare. While aluminum is the most common metal in the crust, and iron only the fourth frequent, the latter holds the distinction of being the most common element on Earth as it forms most of the Earth’s core in one state or another. However, both are examples of very common metals.

The common Earth metals do not really feature in the game, whereas those considered uncommon—arsenic, copper, lead, tin, and zinc—do. Two of Earth’s rare metals—mercury and silver—play parts of differing importance, as do two very rare examples—gold and platinum. Of course, the game also includes the legendary mithral and adamantine.

If you wish to keep the rewards of mining straightforward, consider using Table 4-3. Gems are generally much rarer than ores, being found only as often as the least frequent metals. However, once a vein is discovered, the value of these rare recoverable stones is much greater per 1,000 cubic feet of rock excavated. When a seam or lode is found, roll percentage dice to establish of what the claim primarily consists. There are differences between mines close to the surface and deeper towards the core.

While these are the most common metals extracted, a check through the material components for 5th-to-9th level spells reveals other mineable substances are required, including arsenic, mercury, phosphorus, sulfur (or brimstone), and zinc.

Carbon, manganese and tungsten are also metals that could play a role in game, as together they form wolframite, an armor-piercing material when combined with iron, while iron and carbon make steel. Lime and rare chalks can also be mined and are useful as spell components.

Gemstones tend to fall into two categories: decorative and spell components. Although they overlap considerably, if not entirely, looking again at high-level spell components, the following gemstones are featured:

Agate, amber, black pearl, corundum, crystal (unspecified), diamond, emerald, jacinth, jade, marble, onyx, opal, pearl, quartz, ruby, sapphire, sunstone

Table 4-3: Mined Materials by Region, On or Under the Surface (d%)
Metal Placer1 Surface Crust Outer Mantle Upper Mantle7 Upper/Lower Mantle Zone8
Copper 01-50 01-44 01-30 01-154
Tin 51-75 45-65 31-45 16-255 01-05
Lead 76-89 66-78 46-56 26-334 06-08 01
Iron 90-93 79-86 57-78 34-675 09-57 02-51
Silver 94-96 87-912 79-873 68-814 58-77 52-71
Gold 97-98 92-942 88-913 82-866 78-83 72-78
Platinum 99 95-962 92-943 87-914 84-88 79-85
Gemstones 00 97-982 95-973 92-955 89-93 86-91
Mithral 992 98-993 96-986 94-97 92-96
Adamantine 002 003 99-006 98-00 97-00

1 Materials found in placer mines will usually be low quality. Roll d20-1 and multiply the result by 5 to get a level of purity. For example, if gold is found and a roll of 5 is made on the d20 for a total of 4, the metal is only 20% pure. If a 1 is rolled, the find is worthless no matter what it looks like and may prove to be iron pyrite.

2 These materials range between 30 % and 100% pure. Roll d8+2 and multiply by 10 to get the value.

3 These materials range between 70% and 100% pure. Roll d4+6 and multiply by 10 to get the value.

4 These materials are most often found amongst deposits of other ores.

5 These materials are most often found amongst deposits of rocks.

6 These materials, especially gold, are most often found with deposits of iron.

7 At this depth, the materials are always at least 90% pure. Roll d10+90 to get the value.

8 At this depth, the materials are always at least 100% pure and stand a chance of being more valuable than usual. Roll d20 and on a score of 20 they are worth 10% more than would be expected.

Section 15: Copyright Notice

Survivalist's Guide to Spelunking. © 2021, AAW Games; Thilo Graf, Doug Niles, Stephen Yeardley. Underworld Races and Classes © 2017 AAW Games LLC; Designers: Thilo Graf and Mike Myler

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