At the Earth's Core Edgar Rice Burroughs: Pellucidar Series # 1

The author relates how, traveling in the Sahara desert, he has encountered a remarkable vehicle and its pilot, David Innes, a man with a remarkable story to tell. David is a mining heir who finances the experimental "iron mole," an excavating vehicle designed by his elderly inventor friend Abner Perry. In a test run, they discover the vehicle cannot be turned, and it burrows 500 miles into the Earth's crust, emerging into the unknown interior world of Pellucidar. In Burroughs' concept, the Earth is a hollow shell with Pellucidar as the internal surface of that shell. (wikipedia)