At the Earth's Core: Book 1 of the Pellucidar Series (Bk. 1)

When the inventor Perry fires up his burrowing prospector, it runs out of control, plunging him with his young protege into the centre of the earth. There, instead of being destroyed by the molten lava they expect to find, they discover an inner world of bizarre savagery and unearthly beauty. Here mighty dinosaurs still roam alongside beasts never seen in the world above. And to their horror, they suddenly find themselves enslaved in a land where humans are ruled by the reptilian and evilly intelligent Mahars.