Earthcore by Scott Sigler

Buried deep under a mountain in rural Utah lies the largest platinum deposit ever discovered, worth at least a billion dollars. EarthCore is the only company with the technology to dig a three-mile-long mine shaft, not to mention the guts to go after the mother lode, and EarthCore executive Connell Kirkland is determined to uncover the massive treasure. But at three miles below the surface, where the rocks are so hot they burn bare skin, something has been waiting for centuries. Waiting… and guarding. Kirkland and EarthCore are about to find out firsthand why this treasure has never been unearthed.