Edgar Rice Burroughs 
Savage Pellucidar 
Pellucidar Book 7

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.