Labyrinthian

"To call the Underdark a cave system is as calling Arran, Emperor of Mankind a good swordsman. Technically correct, but doing an injustice to the scale of it all..."

Darin Edelweiss, Commander of the Delvers

Labyrinthian (or the Underdark) is a vast network of caves, caverns and underground lakes as large as seas. It is a labyrinthine web of tunnels either naturally formed or artificially constructed by the various creatures that inhabit this world below. Labyrinthian is vertically divided into three regions:
The Shallows
2-3 miles deep below the surface of Calcaria, the Shallows are not deep enough to form underneath the oceans and are the first point of contact to the surface of the world above. It is here that most of the deep dwelling races inhabit their cities and sometimes trade with outsiders. The Shallows are the least dangerous area of the Underdark, but see the most death as many unprepared adventurers quickly meet their doom here. From this layer many of the old dwarven deep roads can be accesed, though these days the entrances are either sealed shut or fiercely guarded.
The Deeps
Up to 10 miles deep, the Deeps are marred by long, thin and winding tunnels that even go underneath the oceans of the world. Many horrors lurk down here, but an adventurer could theoretically reach any continent of Calcaria through these subterranean passages. It is here that the more unnatural of races have made their home, and those few who return to the surface speak of illithids and beholders in a babbling manner.
Tartarus
Not much is known about the deepest parts of Labyrinthian, ranging anywhere from at least 10 to 30 miles deep, save that it is hot and equally a maze as the layers above. It is said that down here, in Tartarus, are the connecting tunnels to the The Infernum and its devils, though none have so far found it. If the heat does not take you, then other, damnably transcendent creatures will.

Geography

The Underdark's underground landscape varies from small subterranean fissures almost impossible for adventurers to crawl through, to wide expanses of deep water that hide coral caves, hollowed out sections of ice in which creatures live, and places where fungus, bones, or even pure force formed "caves." Many of the old dwarven deep roads can also be found and accessed through the Underdarks cave network.
This varied landscape is as dynamic as the plane of earth. Some areas are always changing over time due to underground magma rivers or the emptying of underground lakes due to cracks in the rock formed by earthquakes. This means maps of the Underdark are highly unreliable and up to date ones are extremely expensive.
The Underdark's environment varies vastly from the surface world, making adventuring through it a dangerous expedition. Just like trekking through a desert, adventurers traversing the Underdark's passages need to be well prepared.
The Underdark is mainly without any source of light save for the occasional luminous rocks or fungi. Large portions of it are also without food or water, making well-stocked food supplies a must-have when exploring uncharted terrain underground.
Lack of light combined with a tomb-like silence made traveling something only for the strong-hearted. The nerve-wracking silence makes even the most experienced adventurers grow agitated after a few days.
Just like deep dungeons, the air can grow stale and even poisonous in the Underdark. It sometimes contains explosive gases from underground volcanic activity or other deadly gases bubbling out of deep fissures in the rock. Bad air and poisonous fumes collect in portions of tunnels and caves more or less isolated from the main vaults and caverns. Siphon-like tunnels holding water can also trap these gases for a long time.

Fauna & Flora

The Underdark is extremely dangerous, especially to those not native to it. There are the usual dangers associated with caverns: claustrophobia, occasional poor air circulation, and getting lost. There is no light except for occasional patches of fluorescent fungi; most Underdark inhabitants either have highly developed senses other than sight or had developed darkvision. Food can be extremely difficult to find, and much of the natural vegetation is poisonous.
In addition to those dangers, the Underdark is also the home of many predators and often-hostile races, including aboleths, beholders, cloakers, derro, deep elves, duergar, dwarves, illithids, kuo-toa, and svirfneblin.
Plant life and animal life can be as varied underground as on the surface. Ranging from small insects to hook horrors or shadow dragons, which set up their realms deep down, everything should be expected. Plant life is unable to utilise sunlight as a source for energy, and thus took on strange forms adapted to life underground. Many fungi are also found in almost every layer of the Underdark.

Alternative Name(s)

The Underdark

Type

Cave System

Inhabiting Species

• Dark Elves