Underdark

A vast subterranean realm inhabited by drow, mind flayers, aboleth, and other strange, sinister creatures. It is a place where few adventurers go and from where even fewer return

".
It extends well past the dungeons made by surface dwellers, encompassing myriad caverns, tunnels and complexes. 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. [source]