Jeff Long's The Descent:
In a cave in the Himalayas, a guide discovers a self-mutilated body with the 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: 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…
And it's sequel, Deeper:
A decade has passed since doomed explorers unveiled a nightmare of tunnels and rivers honeycombing the earth's depths. After millennia of suffering terror and predation, humanity's armies descended to destroy the ancient hordes. Deep beneath the Pacific Ocean, a doomed science expedition killed the subterraneans' fabled leader, and suddenly it seemed that evil was dead and all was right with the world again.
Now "Deeper" arrives to explode that complacency and plunge us back into the sunless abyss. Hell boils up through America's subways and basements to take its revenge and steal our children. Against the backdrop of a looming war with China, a crusade of volunteers races to find the vestiges of a lost race. But a lone explorer, the linguist Ali von Schade, learns that a far greater menace lies in the unexplored heart of the planet. The real Satan can't be killed, and he has been waiting since the beginning of time to gain his freedom. Man and his pitiless enemies are mere pawns in the greatest escape ever devised.