The Caverealm, sometimes also called Underdark or Deepstone, was a vast network of caves and tunnels that lied beneath most of Oshmondu. It could usually be accessed from small tunnels or rifts in mountains and foothills on the surface. This network was distinctly separate from small surface caves, however, and often had only a few small tunnels leading down into the Caverealm proper.

This cave network featured a huge diversity of plants and animals, although not as many and not as location-specific as on the surface. The most common type of plant was fungus. There were also many humanoid races that lived in the Caverealm and built up whole societies, including Khali drow, duergar, and even human states during the Orc Wars. In some surface areas, such as the ice caps and Glacier on Litchy, the Caverealm was closer to the surface and often used as living space by surface races as well.