Subterranean fiction

The genre is based on, and has in turn influenced, the Hollow Earth theory.

In the late 19th century, however, more pseudoscientific or proto-science-fictional motifs gained prevalence.

Common themes have included a depiction of the underground world as more primitive than the surface, either culturally, technologically or biologically, or in some combination thereof.

The former cases usually see the setting used as a venue for sword-and-sorcery fiction, while the latter often features cryptids or creatures extinct on the surface, such as dinosaurs or archaic humans.

A less frequent theme has the underground world much more technologically advanced than the surface one, typically either as the refugium of a lost civilization, or (more rarely) as a secret base for space aliens.