Underdark Geology

A cave is a natural opening in rock that is large enough for creatures to enter. The Underdark, simply put, consists of a linked network of several titanic cave systems. Large portions of it do fit the definition of a “natural opening in rock,” but the Underdark also encompasses areas of deep water that hide coral caves, hollowed-out sections of ice in which creatures live, and places where fungus, bone, or even pure force form “caves.”

Underdark terrain is dynamic and changeable. A map drawn a few decades ago may show tunnels that have long since collapsed, or lakes that are now dry. Terrain can change gradually over the course of decades, centuries, and millennia, or swiftly as a result of an earthquake or volcanic eruption.


The surface world is marked by mighty mountains, high plateaus, and vast plains. The Underdark possesses none of these features, but it does have physical-features all its own. Unlike the surface world, the Underdark is uniquely three-dimensional. Knowing the direction of true north is not enough to navigate the Realms Below, a traveler must also know the depth underground of her destination. It is possible to find the correct coordinates but still be several miles too deep or too shallow.

Many of the Underdark's features can be explained as nothing more than the results of purely physical forces, albeit sometimes on a grand scale. Other features would be unlikely or even impossible in worlds where magic, elementals, planar interstices, and divine caprice did not help to shape the deep places.


Vast, empty voids of awesome scope, Faerun's abysses are rare features that can form insuperable barriers to travel. An abyss is simply a great open space, sometimes many dozens of miles in breadth and virtually bottomless. Some Underdark abysses are scores of miles deep. The difference between an abyss and a vault is difficult to define, but as a general rule, a vast space approachable from its higher reaches is an abyss, while the same space approachable from the floor might be better described as a vault. Abysses tend to be larger and deeper than areas that are considered vaults, but this is not always the case.

Like the starkest mountains of the Lands Above, abysses are often completely impassable to anything without wings. Underdark civilizations near such features sometimes carve out harrowing ledge-paths to circle the tremendous void of the neighboring abyss, or endless stairs to descend its walls.


Perhaps the most common topographical feature of the Underdark, a cave system consists of a series of caverns and passages that may stretch for miles. Caves can be formed by several different methods, but the most common is the action of flowing water. Cave systems often twist, turn, climb, and drop in a maddening fashion, forming three-dimensional mazes that dishearten even the most determined mapmakers.

Caves vary widely in terms of their habitability. Living caves that include streams or rivers are full of life (by Underdark standards) and can often support surprisingly large populations, especially of improbably large and ferocious monsters. Other caves may be barren wastelands, without food or water.


Over the course of a hundred or more centuries, Faerun's deep caverns and vaults have been expanded tremendously by the devling of various Underdark races. Thus, the term dungeon when applied to the Underdark means a structure excavated from the surrounding rock by intelligent creatures. For example, a great duergar city delved into the wall of a vault might be considered a large dungeon, with halls and passageways extending for miles from its entranceways. Dungeon complexes often serve to link two natural features (such as two or more vaults close to each other) with a system of artificial caves that vastly extends the scope of a natural cave system.

Dungeons come in two varieties – abandoned and occupied. Since they are not sources of food or water in and of themselves, empty dungeons do not necessarily attract Underdark settlers. However, dungeons are often supremely well suited for defense, and a dungeon that happened to be near a rich area such as a living cave is almost certain to be occupied by something, even if the original builders are long gone.


Just as on the surface, water can carve deep canyons and gorges in the Underdark. An Underdark gorge is nothing more than a cave that runs vertically instead of horizontally. Gorges often feature streams (and therefore life and food), although the difficulty of the terrain makes a gorge less desirable as a residence than a living cave with less extreme topography.

Since gorges can run for many miles, they often serve as the highways of the Underdark. Travel along the floor of a gorge can be very difficult, but many Underdark races take steps to improve these natural roads for the use of their own merchants and hunters. Gorges also offer good opportunities to change depth and perhaps access another level quickly, through a little climbing.


Water is common in the Upperdark, since runoff from the surface frequently drains into cave systems belowground. In may areas, the water table is close enough to the surface that only the most shallow cave systems can form. However, due to unusual factors involved in the creation of Faerun's Underdark, a water table 20 feet belowground does not necessarily mean that air-filled caves don't exist at greater depths. Planar connections particularly to the planes of Earth and Water, make very unlikely hydrology possible.

Any body of fresh water is called a lake. Underdark lakes range in size from small pools to inland seas hundreds of miles in extent. Large lakes typically occupy either tremendous vaults or connected networks of partially submerges caves. If a lake has both an inlet and an outlet, its water is usually drinkable, but lakes that are not refreshed from time to time may stagnate.

Most lakes are found in the Upperdark and Middledark. Bodies of water that collect in the Lowerdark simply can't drain to any lower elevations, so they tend to be seas (brackish water) instead. However, planar connections to the Elemental Plane of Water mean that at least a fw of the bodies of water in the Lowerdark hold fresh water.

Large lakes can form the best and most accessible highways of the Underdark. In many places, however, the cavern ceiling descends to meet the water, making the lake impossible to all but aquatic creatures.


Unlike gorges, rifts are not formed by erosion. Rather, they are the scars of tremendous upheveals deep in the earth. Rifts are places where vast blocks of stone rose, sunk, or slid past one another in long-ago cataclysms, leaving tremendous chasms. Rifts may be dozens or even hundred of miles in length, and sometimes miles deep, but they are rarely wide – most are less than a bowshot across.

Rifts sometimes break apart preexisting features such as cave systems, presenting formidable obstacles to creatures traveling through caves. In order to continue when faced with a rift, the traveler must climb or descend to the appropriate level on the far side. Like gorges, rifts often serve as vertical highways in the Underdark, offering travelers the opportunity to change depth with little fuss.


Underdark rivers tend to be swift, violent, and tortuous in their windings. It is a rare river indeed the flows level and smooth for more than a few miles at a time before disappearing into a deep gorge or sinkhole in a fuming waterfall. Rivers are the great builders of the Underdark, the natural force that sculpts great caverns and brings lifegiving energy and food to sustain the Underdark ecology. Most rivers are surrounded by a halo of living caves, which can be valuable real estate indeed.


Perhaps the most wondrous of the Underdark's features are the vast, mighty seas of the deep earth. Seas are saltwater bodies, not fresh, and most of them are found in the Lowerdark, though Underdark seas also occur at higher spots beneath Faerun's surface oceans. While air-filled cave systems may extend for dozens or even hundreds of miles beneath the ocean's above, or form air-locked siphons of extraordinary size, these features are exceptional. Most caves beneath bodies of saltwater are simply subterranean extensions of surface oceans.

Seas tend to form in the largest of vaults, occupying caverns large enough to be miniature worlds in their own right. Like the lakes, seas offer some of the best roads in the Underdark, and many are heavily traveled.


Sometimes natural processes form deep pits or wells in the earth. The shaft of such a structure may be carved out by water flowing straight downward in a subterranean waterfall, or created by volcanic activity. Unlike a gorge or a rift, a shaft tends to be a relatively small feature (usually less than a bowshot in diameter), but it may plummet for miles straight down.

Because of their relatively small cross-sections, shafts often serve to channel air movement between disparate portions of the Underdark. In places where the conditions are extreme (for example, a shaft near a superheated magma chamber), the air movement can also be extreme. Screaming winds might roar up or down a shaft in a scouring blast that would put a hurricane to shame. Sometimes, cave systems “breathe” in conjunction with changes in the surface world above, resulting in tremendous rushes of wind in and out through shafts every day.


A tunnel is simply a passage that connects one place with another. Most are cut by creatures, though some are the results of natural movements of the earth and other forces. Underdark races often cut very ambitious tunnels to link multiple cave systems. Though such dreary passageways may be dozens of miles in length, most are only 10 to 20 feet across. Other tunnels are the work of burrowing monsters such as delvers, purple worms, and umber hulks. These “natural” tunnels may be twisting, turning mazes of intersecting passages.

Tunnels are some of the Underdark most useful roads, but they severely restrict a traveler's options. If you don't like where a tunnel leads, you really have no choice to go back the way you came. Tunnels also offer few hiding places for those who cannot blend in with stone, so often they only way to get away from a predator is to run – and hope you're faster.


The higher reaches of the Underdark consist of immense networks of relatively small caves, but as a traveler descends, the number of caves decreases while the size of the individual caves increases. A large cave near the surface may consist of a few dozen linked chambers, each perhaps a few hundred feet long and a few dozen feet wide. But deeper down are openings in the earth that dwarf any surface dweller's conception of a cave.

A typical vault may be 2 to 4 miles across and as much as 1 mile high. Some, however, sprawl for 50 miles or more and reach heights of 5 or 6 miles from the floor. Larger vaults often feature immense columns – huge piers of natural rock that help to buttress the soaring ceiling. Some were formed by unthinkably massive pieces of the world grinding past each other in the very dawn of time, others by the influence of the Underdark's native magic, and still others ny the influence of planar characteristics in buried planar connections. However it was formed, a vault is a world in miniature, with its won streams, lakes, hills, and plateau all contained in a single vast cavern.

Vaults are almost always highly desirable territories, since they usually offer enough space and resources to support huge forests of fungus, moss, and other strange growths. By Underdark standards, most vaults teem with life, so it comes as no surprise that they support the most powerful and numerous of Underdark settlements. Some stories even tell of illuminated vaults, places where sun-bright crystals in the ceilings blaze with the intensity of true daylight and support green pants and surfacelike fauna in abundance.


It is not universally true in Faerun that descending 40 to 50 miles straight down in any spot brings a traveler to magma. Volcanic activity is extremely variable in the Underdark. Isolated pools of magma seep up almost to the surface in all sorts of places without any other volcanic activity, and in other places deep tunnels and vaults support humanoid settlements at depths where magma should be all that's present. Again, planar anomalies, deific intervention, and the powerful magic of the earth itself are likely to blame. Whatever the cause of these surprising conditions, racing rivers of molten rock, caverns full of brimstone and sulfurous reek, and scalding geysers and hot springs can be found at almost any depth in the Underdark. Underdark volcanoes aren't really mountains – they are usually tremendous fissures or magma chambers that can vomit deadly rivers of lava into nearby caverns with little or no warning.

Rocks and Rock Formations

All the topographical features of the Underdark share one thing in common: They're surrounded by rock. Rock in its various forms is every bit as relevant to the denizens of the Underdark as weather is to surface folk. Understanding the significance of different sorts of rocks often means the difference between life and death for those who live in the Realms Below.

Rock are divided into four categories: sedimentary, igneous, metamorphic, and magicmorphic. These labels describe how the rock was formed.

Sedimentary Features

Sedimentary rock formed from softer clays and deposits of organic matter long ago. These materials were compacted into discrete layers in the same sort of process that turns snow into glacial ice. Sedimentary rock is soft and easily weathered, which means that flowing water can gouge vast cave systems from it in a relatively short time, geologically speaking.

Many of the classic cave features, such as stalagmites and stalactites, are found in caves formed from sedimentary rock. Such features are called solution-based, or dripstone, formations because they form from the slow motion and evaporation of water that contains dissolved minerals.

A solution-based rock formation is considered “living” if it is still in the process of forming. With enough patience and time, the growth patterns of such formations can be shaped, although it may take dozens of years for such alterations to become evident. Some Underdark races, such as the drow and the svirfneblin, occasionally shape growing deposits into particular forms.

Igneous Features and Rocks

Igneous features are formed or influenced by fire – usually magma chambers and flows, although openings into the Elemental Plane of Fire are not uncommon in many parts of the Underdark.

Metamorphic Rocks

Metamorphic rocks occur where intense heat and pressure act to harden rock and alter its consistency. Marble is a good example of a sedimentary rock (limestone) that volcanic activity has transformed into a much harder stone. Metamorphic rock formations are similar to those of the premetamorphic rock, though often compressed. Caves formed from metamorphic rock more often take form of faults, fissures, or similar discontinuities caused by movement of adjacent slabs of stone.

Many metallic ore deposits from in metamorphic settings. Some valuable minerals, such as garnet and tourmaline, also form only in such environments. Hence mining-orientated cultures, such as the dwarves, tend to settle in areas that feature metamorphic rocks.

Since metamorphic stone is often dense, hard, and beautiful, some varieties are highly regarded as building materials. Many huge quarries can be found in the depths of the Underdark.

Magicmorphic (Magic-Formed Rocks)

Magicmorphic Rocks are stones or rock formations that have been warped and changed by magic. The Underdark of Faerun includes several very different manifestations of magicmorphic rocks.

Nonrock Tunnels

Only a newcomer to the Underdark would be naive enough to think that Toril's deep caverns are formed exclusively of rock. A variety of other material can form these sorts of features: coral, ice, glacial, bones, force, and fungi.