Savage Pellucidar

Two explorers from the Earth's surface had carved themselves an empire at the Earth's core - Pellucidar. The dangers were many. Untamed tribes and vicious beasts were a constant peril, and there were times when the lives of the surface-men were at stake. But, just when things seemed hopeless, they came up with a trump card - a new weapon. What they didn't realize was that it was a weapon that could bring its makers to the brink of disaster.