At the Earth's Core, by Edgar Rice Burroughs. Part IV.

David Innes is a mining heir who finances the experimental "iron mole," an excavating vehicle designed by his elderly inventor friend Abner Perry. In a test run, they discover the vehicle cannot be turned, and it burrows 500 miles into the Earth's crust, emerging into the unknown interior world of Pellucidar. In Burroughs' concept, the Earth is a hollow shell with Pellucidar as the internal surface of that shell.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. Innes and Perry are captured by the Mahars' ape-like Sagoth servants and taken with other human captives to the chief Mahar city of Phutra. Among their fellow captives are the brave Ghak, the Hairy One, from the country of Sari, the shifty Hooja the Sly One and the lovely Dian the Beautiful of Amoz. (From Wikipedia.)