Pellucidar

Pellucidar was a location from various Edgar Rice Burroughs books; including the comic, Tarzan at the Earth's Core. It was an underground world which features prehistoric fauna including dinosaurs, Ice Age fauna, sabre-toothed tigers, Megatheriums, Phorusrhacoses, pterosaurs, plesiosaurs, ichthyosaurs, Dimetrodons, cave bears, and Neanderthals.

According to the Seventh Doctor, Pellucidar was inspired by the capital city of the Silurians. (AUDIO: The Silurian Candidate)