At the Earth's Core and Pellucidar by Edgar Rice Burroughs [Pellucidar #1-2]

AT THE EARTH'S CORE and PELLUCIDAR are the first two novels in Edgar Rice Burroughs' Pellucidar series. These classics of "hollow Earth" fiction tells of two unwilling adventurers who, through a mining mishap, burrow into the subterranean world of Pellucidar, 500 miles below the Earth's surface. Pellucidar is inhabited by prehistoric creatures of all geological eras, and dominated by the Mahars, a species of flying reptile both intelligent and civilized, but which enslaves and preys on the local stone-age humans.