Edgar Rice Burroughs 
Pellucidar 

Pellucidar is the second novel set in Burroughs‘ fictional land of the same name, beneath the earth’s crust. David Innes returns to Pellucidar from the surface, in search of his friend and colleague Abner, as well as his love, Dian the Beautiful. He must deal with the conflicts following their initial discovery of Pellucidar, and fight for the new, human civilization being built there. In later novels, various other protagonists enter Pellucidar, including Burroughs‘ most famous character, Tarzan.