The Underdark

The region called the Underdark is an entire world beneath the feet of the surface dwellers of Faerûn. Inhabited by monstrous and evil creatures that shun the daylight, the Underdark teems with entire cities and nations of derro, drow, duergar, and mind flayers (illithids). It is also home to even stranger races such as aboleths, beholders, and kuo-toas, as well as slaves of just about any intelligent humanoid found on the surface.
These evil beings battle or trade with each other for resources, magic, and power, forming alliances that collapse when plots unravel or better opportunities come along. Interspersed with the warring city-states are enclaves of gnomes, svirfneblin, dwarves, and other neutral and good folk, who remain isolated or resist encroachment by malign neighbors.

Geography

Because most of the Underdark has been only cursorily surveyed, and given that the largest caves are only a few miles long, few geographical features would be considered noteworthy to a surface dweller. Lakes tend to be the largest features, although the nearby stone may dip below the surface of the water, breaking it into quasi-separate regions that can only be identified as the same body of water by the most meticulous cartographer.
Tunnels in the Underdark extend for miles, some ballooning into caverns thousands of feet across, only to shrink to narrow spaces too small for a halfling to squeeze through. The largest cavern halls become representations of the surface in miniature, with hills, valleys, underground rivers, and lakes. In this three-dimensional environment, most races make use of the walls and ceilings of their caverns, accessing the higher levels with natural or magical flight or levitation or wall-crawling mounts such as giant spiders or certain breeds of lizards.
The Underdark is divided into three general levels. The upper Underdark is close to the. surface, has considerable interaction (trade, raids, or conquest) with surface races, and is mainly inhabited by drow, beholders, dwarves, mind flayers, svirfneblin, and - under cities -wererats. Water and food are relatively abundant, and adaptation to darkness is mild.
The inhabitants of the middle Underdark tend to see surface races as slaves. These include drow, lone aboleths, cloakers, derro, mind flayers, svirfneblin, and kuo-toas. Water and food are difficult to find.
The lower Underdark is incredibly strange, filled with alien societies and bizarre cultures, hostile to any unlike them, ruled by aboleths, cloakers, derro, and mind flayers. Food and water are very rare, so the races here prey upon each other for survival. Adaptation to darkness is often extreme, with new and peculiar senses appearing in some monsters. All levels contain evil humanoids, usually as slaves in civilized cities.

Type

Underground / Subterranean