Pellucidar

Pellucidar was an underground world with prehistoric animals from various Edgar Rice Burroughs books.

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

Trivia

• Pellucidar was made by the same writer as Tarzan, the feral man who occasionally went there; most notably including the novel Tarzan at the Earth's Core and two episodes of the Walt Disney series The Legend of Tarzan.
• Pellucidar features many prehistoric fauna which've also made some appearances in Doctor Who including dinosaurs, mammoths, mastodons, sabre-toothed tigers, Megatheriums, Phorusrhacoses, pterosaurs, plesiosaurs, ichthyosaurs, Dimetrodons, cave bears, and Neanderthals.