Underdark

The Underdark was the vast network of underground caverns and tunnels underneath the surface of Terra. It was home to a host of evil beings driven deep into the caverns in the Ninium Period, and has been since.
There are 3 layers of the Underdark:
Upperdark: The first three miles below the surface. It was here where the surface dwellers and those in the Underdark most often met.
Middledark: Located three to ten miles below the surface, this layer was where most of the Underdark cities were located.
Lowerdark: Ten miles or more below the surface, the Lowerdark was where even those who knew the Underdark were loath to go.

Geography

The Underdark's underground landscape varied from small subterranean fissures almost impossible for adventurers to crawl through, to wide expanses of deep water that hid coral caves, hollowed out sections of ice in which creatures lived, and places where fungus, bones, or even pure force formed "caves."
This varied landscape was as dynamic as the surface of Terra. Some areas were always changing over time because of underground magma rivers or the emptying of underground lakes due to cracks in the rock formed by earthquakes. This meant maps of the Underdark were highly unreliable.

Domains

The Underdark was not one giant cavern under Terra, but rather, many huge networks of caverns and caves. As a result, it was not always possible to travel from one end of the Underdark to the other. The Underdark was divided into several domains that were similar to continents of the world above. While it was possible to travel from one place to another within a domain, separate domains tended to have very few passages linking them. The major domains of the Underdark were the Buried Realms, the Darklands, the Deep Wastes, the Earthroot, the Glimmersea, Great Bhaerynden, the Northdark, and Old Shanatar.

Natural Resources

The Underdark's environment varied vastly from the surface world, making adventuring through it a dangerous expedition. Just like trekking through a desert, adventurers traversing the Underdark's passages needed to be well prepared.
The Underdark was mainly without any source of light save for the occasional luminous rocks or fungi. Large portions of it were 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 made even experienced adventurers grow agitated after a few days.
Just like deep dungeons, the air could grow stale and even poisonous in the Underdark. It could contain explosive gases from underground volcanic activity or other deadly gases bubbling out of deep fissures in the rock. Bad air and poisonous fumes could collect in portions of tunnels and caves more or less isolated from the main vaults and caverns. Siphon-like tunnels holding water could also trap these gases for a long time.

Climate

The temperature underground varied according to depth and the proximity of thermal activities in the area. Throughout the year, the temperature remained stable and moderate in most regions, usually slightly clammy or chilly. The water underground was extremely cold and posed a hypothermia risk.

Type

Underground / Subterranean

Location under

Terra