At the Earth's Core - Edgar Rice Burroughs

This is the first book in the Pellucidar series. 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. (adapted from Wikipedia)