Underdark The Underdark was the vast network of underground caverns and tunnels underneath the surface of Abeir - Toril . It was home to a host of evil beings driven deep into the caverns at the end of the age of demon s . Geography . Domains . The Underdark was not one giant cavern under Faerûn , 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 . Topography . 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 Abeir - Toril . 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 . Environment . 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 . Inhabitants . The Underdark was extremely dangerous , especially to those not native to it . There were the usual dangers associated with caverns : claustrophobia , occasional poor air circulation , and getting lost . There was no light except for occasional patches of fluorescent fungi ; most Underdark inhabitants either had highly developed senses other than sight or had developed darkvision . Food could be extremely difficult to find , and much of the natural vegetation was poisonous . In addition to those dangers , the Underdark was also the home of many predators and often - hostile races , including beholder s , derro , drow , duergar , dwarves , illithid s , kuo - toa , and svirfneblin . Ecology . Plant life and animal life could be as varied underground as on the surface . Ranging from small insects to hook horror s or shadow dragon s , which set up their realms deep down , everything could be expected . Plant life was unable to utilize sunlight as a source for energy , and thus took on strange forms adapted to life underground . Many fungi were also found underground . Being unable to digest normal nutrients most of the time , they relied on absorbing " faerzress " for their energy .