Pellucidar — a series by Edgar Rice-Burroughs

David Innes and Abner Perry build a giant mechanical prospector with which they hope to uncover vast mineral deposits far beneath the surface. On the “Iron Mole’s” first trip, however, they discover that their vehicle can’t be steered. Death seems certain, for doesn’t everyone know that the center of the Earth is a molten mass of white-hot magma?