Pellucidar by Edgar Rice Burroughs

Pellucidar is a science fiction novel written by Edgar Rice Burroughs and is the second novel in the series set in the fictional Hollow Earth land of Pellucidar, following on from the events in "At the Earths Core".

The plot involves the use of an "iron mole" to burrow deep into the earths crust and an encounter with a "hollow earth", this hollow earth resides on an inner surface of the earth, and mirrors that of the "outer earth" reversed with a greater landmass than water and a sun in the center.

The inner earth inhabitants include primitive prehistoric animals, including dinosaurs, intelligent flying reptiles and stone age humans.