At the Earth's Core by Edgar Rice Burroughs, Science Fiction, Literary

The invention of a drilling vehicle leads Abner Perry and his friend David Ennis on an adventure into the earth, uncovering the fantastical inner world of Pellucidar. This realm, where the sun is actually the molten core of the planet, is teeming with dragons, apes, and other bizarre creatures. As they navigate this vibrant landscape, they encounter sorcerous royalty, including a princess who falls for Ennis, plunging them into a thrilling escapade filled with danger and excitement.