The World Cavern is a vast subterranean realm inhabited by dark elves, mindflayers, formorians, and other strange, sinister creatures. It is a place where few humans go and from where even fewer return. It extends well past the dungeons made by surface dwellers, encompassing myriad caverns, tunnels and complexes.

The World Cavern's physical characteristics are similar to conditions in surface world caverns deep underground, except at immense size, however, it is permeated with a mysterious form of wild magic, which is used as a source of energy by the native plant life which interferes with scrying and teleportation spells. The World Cavern is extremely hazardous, especially to non-native creatures and the sheer depth and ambient wild magic has strange effects on the plants and creatures who spend too much time down there. There are the usual dangers associated with caverns: claustrophobia, occasional poor air circulation, and getting lost, cave floor and ceiling collapses. Being eaten or enslaved by psychic monsters from beyond time is also a hazard.

There is no light except for occasional patches of phosphorescent fungus; most World Cavern inhabitants either have highly developed senses other than sight or have developed enhanced darkvision. Food can be extremely difficult to find, and much of the natural vegetation is poisonous. In addition, potable water is hard to locate.