Pellucidar

Pellucidar is the fictional internal surface of a 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 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.