Subterranean fiction and the Hollow Earth theory are based on what kind of concept?
Subterranean fiction: Subterranean fiction is a subgenre of adventure fiction or science fiction which focuses on underground settings, sometimes at the center of the Earth or otherwise deep below the surface. Subterranean fiction:  The genre is based on and has in turn influenced the Hollow Earth theory. Hollow Earth: The Hollow Earth is a pseudoscience concept proposing that the planet Earth either is entirely hollow or otherwise contains a substantial interior space.
a pseudoscience