We are not alone. Some call them devils or demons. But they are real. They are down there. And they are waiting for us to find them.

In a cave in the Himalayas, a guide discovers a self-mutilated body with a warning: Satan exists. 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: the underworld is a vast geological labyrinth populated by another race of beings.

With all of Hell's precious resources and territories to be won, a global race ensues. Nations, armies, religions, and industries rush to colonize and exploit the subterranean frontier. A scientific expedition is launched westward to explore beneath the Pacific Ocean floor, both to catalog the riches there and to learn how life could develop in the sunless abyss. But in the dark underground, as humanity falls away from them, the scientists and mercenaries find themselves prey not only to the savage creatures, but also to their own treachery, mutiny, and greed. One thing is certain: Miles inside the earth, evil is very much alive.