Subterranea is the vast world located under the surface of the Earth. It is ruled by the Mole Man and it's main inhabitants are the Subterraneans. There are locations on the surface that lead to Subterranea, such as Monster Island.
Subterranea has Moloid cities such as :
Lechuguilla, which is built along the walls of a deep cavern.
Meramec, the underwater city inhabited by aquatic Moloids.
Abandoned City of the High Evolutionary, build deep in Subterranea, it has recently been raised to the surface world.