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
Underdark, offering travelers the opportunity to change depthwith little fuss.
Rivers
Underdark rivers tend to be swift, violent, and tortuous in theirwindings. 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
Underdark ecology. Most rivers are surrounded by a halo ofliving 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
Underdark seas also occur at higher spots beneath Faerûn’s surfaceoceans. 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
Underdark. In places where the conditions are extreme (forexample, 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
umber hulks. These “natural” tunnels may be twisting, turningmazes 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.