Wikipedia
Pellucidar is a fictional Hollow Earth invented by Edgar Rice Burroughs for a series of action adventure stories. In a notable crossover event between Burroughs' series, there is a Tarzan story in which the Ape Man travels into Pellucidar.
The stories initially involve the adventures of mining heir David Innes and his inventor friend Abner Perry after they use an "iron mole" to burrow 500 miles into the Earth's crust. Later protagonists include indigenous caveman Tanar and additional visitors from the surface world, notably Tarzan, Jason Gridley, and Frederich Wilhelm Eric von Mendeldorf und von Horst.
Pellucidar is a 1915 fantasy novel by Edgar Rice Burroughs, the second in his series about the fictional " hollow earth" land of Pellucidar. It first appeared as a four-part serial in All-Story Weekly from May 8–29, 1915. It was first published in book form in hardcover by A. C. McClurg in September, 1923. A map by Burroughs of the Empire of Pellucidar accompanied both the magazine and book versions.