At the Earth's Core
by Edgar Rice Burroughs
Burroughs set this classic Scifi/Fantasy novel in a version of the Hollow Earth which can only be reached by an advanced tunneling machine. Breaking through into the inner core, the adventurers find a world (Pellucidar) which physically resmbles Cyrus Teed's Cellular Cosmology, except that we are on the outside. Inside is a primitive world where dinosaurs still exist, and humans are not at the top of the food chain...