Pellucidar

David Innes and his captive, a member of the reptilian Mahar race of the interior world, return from the surface in the Iron Mole created by his friend Abner. Emerging in Pellucidar at an unfamiliar location, David frees his captive. He names the place Greenwich and uses the technology he has brought to begin the orderly exploration and mapping of the unknown land while searching for his lost companions. He soon meets and befriends a new ally, Ja the Mezop of the island country of Anoroc; later he finds Abner, from whom he learns that in his absence the human rebellion against the Mahars has not been going well...