The story begins with a new-age expedition into the mountains of Nepal. 
The explorers become trapped in a mountain cave during a freak snowstorm. Within the cave, they discover a mutilated and mummified corpse. They soon discover that they are not alone in the warren of caves that wind through the mountain. The opening is quite scary and horrific, it's easy to feel that this segment inspired the British horror movie of the same name.
After the opening section, there is an abrupt shift in the tone and feel of the story. It becomes a tale of adventure, colonization, and conquest. This was a bit jarring at first, but soon I was swept up in the journey. There are two major factions intent on the underground world: Helios (a mega-corporation, with much of the evil that tends to imply in fiction) and the Beowulf group (a group of scholars seeking the physical source for the legend of Satan).
Helios has sent down an invading force to claim all the profitable resources below the earth's surface. It is largely militaristic and focused on wiping out the native populace, called "Hadals" or "Haddies". The Hadals defend their territory, but are largely outnumbered. Suddenly their attacks on the invaders stop and they seem to have disappeared. Some flee deeper into the earth, others begin surfacing in our world.
The Beowulf group manages to infiltrate the Helios expedition in the form of a young nun named, Ali van Schade. None of the groups are physically healthy enough to make the journey themselves. Ali's specialty is linguistics and she has been trying to find the first language. They discover a remarkable number of similarities between the hadals and humans as well as the obvious differences.
There is much complexity in the story's plot and the mythos provided. The exploration of the consequences of finding the "literal Hell" are interesting as well. Long manages to surprise the reader with several plot twists. His descriptions of the world underneath the world are frequently breathtakingly beautiful. I look forward to reading the sequel.
Happy reading!
