Pellucidar is a fictional Hollow Earth invented by American writer Edgar Rice Burroughs for a series of action adventure stories. In a crossover event, Tarzan, who was also created by Burroughs, visits Pellucidar.

The Hollow Earth is a concept proposing that the planet Earth is entirely hollow or contains a substantial interior space. Notably suggested by Edmond Halley in the late 17th century, the notion was disproven, first tentatively by Pierre Bouguer in 1740, then definitively by Charles Hutton in his Schiehallion experiment around 1774.