The Underdark was the enormous system of subterranean caves, caverns, and tunnels located beneath the surface of the world. Like many natural formations on the Prime Material, it had counterparts in the Vivid Realms and the Umbral Realms. Given its size and scope, it was not always possible to travel from one end of the Underdark to the other; it was divided into several domains that were similar to the landmasses of the world above.
The Underdark's underground landscape varied from small subterranean fissures almost impossible for Mortalkind to crawl through to more conventional caves and caverns. There were also wide expanses of deep water in which coral caves grew, blocks of ice that creatures hollowed out and dwelt inside, and cave-like spaces within masses of fungus, bone, or bubbles of magical force.
This varied landscape was as dynamic as the surface of the Head of the Greatwyrm. Some areas were always changing due to the shifting of underground magma rivers or the emptying of underground lakes from earthquakes cracking the rock. This meant that the scant existing maps of the Underdark were highly unreliable.
Denizens of the Underdark referred to all the surface as the "World Above," and categorized all below it into three divisions: the Upperdark, the Middledark, and the Lowerdark.