Pellucidar15 books in series
At the Earth's Core Publisher's Summary
Convinced of the worthiness of a giant, earth-boring machine, David Innes becomes a willing participant in an experiment. On the machine's maiden attempt at boring through the Earth, David and inventor Perry are trapped in the control room as the machine turns into a runaway. It bores steadily down toward the center of the Earth until finally it penetrates into a strange, subterranean world. The two travelers are immediately welcomed by a huge bear-like creature interested solely in having them for breakfast.