Back to the Stone Age by Edgar Rice Burroughs

Five hundred miles beneath the surface of the Earth lies another world – a world of eternal day and endless horizons, in which dinosaurs still roam and cavemen hunt and terrors forgotten in the outer world still survive. Lieutenant von Horst, member of an exploring expedition, was left behind in this lost land of Pellucidar. Back To The Stone Age is the thrilling story of his perilous adventures, along with the cavegirl he loved, in that primitive unknown world.