Subterranean Fiction

Subterranean fiction is a subgenre of speculative fiction that focuses on fictional underground settings, sometimes at the center of the Earth or otherwise deep below the surface, and is based on, and has in turn influenced, the Hollow Earth theory. The earliest works in the genre were Enlightenment-era philosophical or allegorical works in which the underground setting was largely incidental, but in the late nineteenth century more pseudoscientific and proto-science-fictional motifs gained prevalence, commonly depicting the underground world as more primitive than the surface, culturally, technologically or biologically, and populated by cryptids or creatures extinct on the surface such as dinosaurs or archaic humans, though a less frequent theme instead makes the underground world more technologically advanced, as the refuge of a lost civilization or a secret base for space aliens.