Underdark

A vast region of underground caverns and tunnels miles beneath the surface of the planet, home to a great array of beings and whole cities and kingdoms of creatures like Drow, ahu-nixta, myconids, ratfolk, undead, norkers, and illithid. Conditions vary, but for the most part, the Underdark is cold, dark, wet, and silent, covered in molds, fungi, and luminous plants.

Sages commonly divide the subterranean regions of Jesenreth into these sections:

The Upperdark, starting around 500 feet beneath the surface, down to one mile. Ratfolk and norkers are the major powers here, with outposts and raiding parties from the lower regions.

The Underdark (sometimes called locally the Middle Dark), starting around a mile down to 10 miles down. This region contains the great kingdoms and cities, including that of the drow, the Darakhul Empire, and the ahu-nixta, and smaller communities of duergar, derro, and svirfneblin.

The Lower Dark, from 10 miles to an unknown depth, possibly 30 miles. A strange and alien realm ruled by aboleth, illithids, and myconids.

Below this even the most learned sages have little firm information, but there are stories, such as the Darksea and the Center World.