At the Earth's Core

At the Earth's Core is a 1914 fantasy novel by Edgar Rice Burroughs, the first in his series about the fictional "hollow earth" land of Pellucidar. It first appeared as a four-part serial in All-Story Weekly from April 4–25, 1914. It was first published in book form in hardcover by A. C. McClurg in July, 1922.

Short summary:

The author relates how, traveling in the Sahara desert, he has encountered a remarkable vehicle and its pilot, David Innes, a man with a remarkable story to tell. David Innes is a mining heir who finances the experimental "iron mole," an excavating vehicle designed by his elderly inventor friend Abner Perry. In a test run, they discover the vehicle cannot be turned, and it burrows 500 miles into the Earth's crust, emerging into the unknown interior world of Pellucidar. In Burroughs' concept, the Earth is a hollow shell with Pellucidar as the internal surface of that shell.