Topography

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.

Abysses

Vast, empty voids of awesome scope, Faerûn’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.

Caves

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.

Dungeons

Over the course of a hundred or more centuries, Faerûn’s deep

caverns and vaults have been expanded tremendously by the

delving 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 happens 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.

Gorges

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.

Lakes

Water is common in the Upperdark, since runoff from the surface

frequently drains into cave systems below ground. In many

areas, the water table is close enough to the surface that only the

most shallow cave systems can form. However, due to the

unusual factors involved in the creation of Faerûn’s Underdark,

a water table 20 feet below ground 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 submerged caves. The Lake

of Shadows and the Giant’s Chalice are examples the former

type, and the Darklake is an example of the latter. 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 or 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 few 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 impassable to all but

aquatic creatures.

Rifts

Unlike gorges, rifts are not formed by erosion. Rather, they are

the scars of tremendous upheavals 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 hundreds of miles in length, and

sometimes miles deep, but they are rarely very 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

with little fuss.

Rivers

windings. It is a rare river indeed that 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 life-giving energy and food to sustain the

living caves, which can be valuable real estate indeed.

Seas

Perhaps the most wondrous of the Underdark’s features are the

vast, nighted seas of the deep earth. Seas are saltwater bodies, not

fresh, and most of them are found in the Lowerdark, though

oceans. While air-filled cave systems may extend for dozens

or even hundreds of miles beneath the oceans above, or form air-locked

siphons of extraordinary size, these features are exceptional.

Most caves beneath large 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.

Shafts

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

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.

Tunnels

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 or 20 feet across. Other tunnels are the

work of burrowing monsters such as delvers, purple worms, and

mazes of intersecting passages.

Tunnels are some of the Underdark’s 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 but to go back the way

you came. Tunnels also offer few hiding places for those who

cannot blend in with stone, so often the only way to get away

from a predator is to run—and hope you’re faster.

Vaults

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 by the confluence of planar characteristics

in buried planar connections. However it was formed, a vault

is a world in miniature, with its own streams, lakes, hills, and

plateaus 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 plants and

surface-like fauna in abundance.

Volcanoes

It is not universally true in Faerûn that descending 40 or 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.