Underdark

The Underdark is a sort of demiplane under the ground of the Material Plane. It houses many humanoids, such as drow, duergar, kuo-toa, kobolds, and quaggoths, but it is not really all that uncommon for mind flayers or demons to enter this subterranean realm.

Description
The Underdark is quite large. In fact, it is almost as big as the Material Plane. The spelunkers that come here, almost never get out, as it is a maze of grottoes and caves. Most people think of total darkness when they hear Underdark, which is understandable when it’s literally in the name, but in fact, many beautiful places can be found here. Forests of luminescent fungi, great cities of the duergar, or ancient shrines made by the saurians, it is truly wonderful for any curious mind.