Pellucidar

Pellucidar is a fictional Hollow Earth milieu invented by Tarzan creator 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 cave man Tanar and additional visitors from the surface world, notably Tarzan, Jason Gridley, and Frederich Wilhelm Eric von Mendeldorf und von Horst.