In the Kalahari Desert, a nun unearths evidence of a proto-human species and a deity called Older-than-Old. In Bosnia, something has been feeding upon the dead in a mass grave. So begins mankind’s most shocking realization: that the underworld is a vast geological labyrinth populated by another race of beings. Some call them "devils" or "demons." But they are real. They are down there. And they are waiting for us to find them…