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

This Halcyon Classics ebook is the first of Edgar Rice Burroughs' Pellucidar series, AT THE EARTH'S CORE.

A classic of "hollow Earth" fiction, AT THE EARTH'S CORE 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.