The underworld is an alternate reality, inhabited by monsters, forbidden to humans, and the place where most of the events of Casper's Scare School stories take place.

The Underwold is a parallel universe, unknown for most of humans, this dimension is ruled by the mighty Kibosh, who is respected and feared by all the dwellers who reside on there, This world can be accessed only through portals in the human world and by special ships designed to cross between dimension, being the Scare School's Ghost ship the main example. Little is known about the underworld, and only a few places in it where seen including Scare School, the Forbidden forest and the underworld ocean.