Subterranean fiction

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Subterranean fiction is a subgenre in fiction which mostly focuses on an underground world, sometimes at the center of the Earth or otherwise deep underground. It is heavily based on, and influences, the Hollow Earth theory. The most prevalent themes are that the underground world is more primitive than that of the surface, with such components as dinosaurs, cavemen and ancient creatures; or alternately, that the underground world is far more advanced than the surface, most typically due to alien presence.