I'm currently reading The Descent - Jeff Long

Amazon.com
In a high Himalayan cave, among the death pits of Bosnia, in a newly excavated Java temple, Long's characters find out to their terror that humanity is not alone--that, as we have always really known, horned and vicious humanoids lurk in vast caverns beneath our feet. This audacious remaking of the old hollow-earth plot takes us, in no short order, to the new world regime that follows the genocidal harrowing of Hell by heavily armed, high-tech American forces. An ambitious tycoon sends an expedition of scientists, including a beautiful nun linguist and a hideously tattooed commando former prisoner of Hell, ever deeper into the unknown, among surviving, savage, horned tribes and the vast citadels of the civilizations that fell beneath the earth before ours arose. A conspiracy of scholars pursues the identity of the being known as Satan, coming up with unpalatable truths about the origins of human culture and the identity of the Turin Shroud, and are picked off one by bloody one. Long rehabilitates, madly, the novel of adventures among lost peoples--occasional clumsiness and promises of paranoid revelations on which he cannot entirely deliver fail to diminish the real achievement here; this feels like a story we have always known and dreaded. --Roz Kaveney, Amazon.co.uk --This text refers to an out of print or unavailable edition of this title.

From Publishers Weekly
The premise of this millennial thriller is as audacious as it is problematic: "if there can be a historical Christ," one character hypothesizes, "why not a historic Satan?" Demystification of the ultimate Bad Guy is no easy feat, but Long (Angels of Light) brings it off, if just barely, in a dizzying synthesis of supernatural horror, lost-race fantasy and military SF. From the experiences of a varied cast of charactersAincluding Sister Ali, a Catholic nun serving in South Africa, and Elias Branch, a major with NATO forces in BosniaAa 21st-century think tank calling itself the Beowulf Circle distills a startling theory: The biblical Satan and his devils in Hell are mythic renderings of Homo hadalis, grotesquely malformed offshoots of Homo sapiens who for centuries have surfaced from underground hideouts to prey on human beings. With the help of Ike Crockett, an escapee from 10 years of "hadal" captivity, Beowulf infiltrates the Helios Corporation's mission to explore caverns honeycombing Earth's interior. Once beneath the Mariana Trench, Beowulf discovers that Helios intends to forcefully annex the world inside the earth's crust to further its business ambitions. Meanwhile, topside, Beowulf's theologians and metaphysicians surmise that the elusive "Satan" has evolved a human form to pass secretly among mankind. Like the subterranean trail blazed by its adventurers, the narrative twists, turns, dead-ends and backtracks. Inventive scenes of underground wonders alternate with talky stretches of scientific discourse and mawkish moments of romance between Ike and Ali. Though its devils prove disappointingly to be made in the image of humans, Long's novel brims with energy, ideas and excitement. 150,000 first printing; major ad/promo; film rights sold to Warner Bros. (July)
Copyright 1999 Reed Business Information, Inc. --This text refers to an out of print or unavailable edition of this title.

http://www.amazon.com/Descent-Jeff-Long/dp/051513175X