Deeper, by Jeff Long, is a sequel to Descent that came out a few years ago. I enjoyed Descent, though it was dark, morbid, and creepifying. Deeper is too. And gory (I don't recall if Descent was gory).
Deeper takes places 10 years after Descent. The huge expanse of the underworld discovered in Descent is being colonized and exploited in Deeper. But the Subterrain still has an unwelcome effect on most people that spend any time at all below: it can change you physically in the form of physical mutations or just plain drive you crazy.
The residents of the underworld, the Hadals, were thought eradicated in Descent. Some have survived and have kidnapped groups of children from across America on Halloween.
The novel follows two stories: one mother's quest to form an army and recover her beloved daugher, and the only survivor from the first book is searching for the "king" of the underworld, what she thinks is the true Satan.
In between those two main stories, the book also gives us glimpes at a brewing Surface war with China, and conversations between that leader of the underworld and one of his captives. Those conversations hint that this king really is the devil that has lived thousands of years and tinkered with our world from his hiding spot in the bowels of the earth.
As I mentioned, this is a dark book. And gloomy. I'm not sure what the point of the story is. We start out in one place, and apart from the war with China, pretty much end up at just about the same place with no real resolution to anything. It's a quest story where nothing happens besides people dying and going crazy. There's no happy ending here. Although... now that I think about, the main character - Ali from the first book - wants to know what the symbols in the caves mean, and she gets to a point at the end where she can finally spend time figuring them out.
If you like horror/suspense/adventure novels. You might like Deeper, but it's a little too dark for me.