At the Earth's Core. Pellucidar Book 1

When Abner Perry invents a vehicle that essentially drills through the earth, he takes it to his good friend (and independently wealthy man about town) David Ennis. And what else can they do? Drill down into the earth, of course. What they find there isn't what we'd expect: it's an inner world called Pellicidar, a place where the sun neither sets nor rises - because what appears to be the sun is no sun at all, but the molten core of the earth.
Pellucidar is a great fun fantasy world, full of dragons, apes, and reptiles and Weird Things. It's ruled by sorcerous royalty (the princess falls in love with Our Hero, of course) and of course our heroes end up hip-deep in dragons...