Pellucidar — Edgar Rice Burroughs

In the hidden world at the Earth's core, David Innes who first discovered it, was struggling to carve a civilization out of its Stone Age perils, but he had to drop his work to embark on a hunt for the kidnapped empress, the cavewoman Dian the Beautiful. His search for Dian, his use of surface science against Pellucidar monsterdom, makes Edgar Rice Burroughs' PELLLICIDAR an action-packed high-adventure science-fiction novel.