Librivox: Pellucidar by Burroughs, Edgar Rice

Summary: Pellucidar is a fictional "Hollow Earth" milieu invented by Edgar Rice Burroughs for a series of action adventure stories. The stories initially involve the adventures of mining heir David Innes and his inventor friend Abner Perry after they use an "iron mole" to burrow 500 miles into the earth's crust. This is the second book in the series. (adapted from Wikipedia)