The characters in this book are very well realized, with backgrounds and realistic motivations. The storyline starts off very grounded in reality, but still very interesting. Gradually, though, things happen that make you realize there is more going on than you expected, until by the end things become quite fantastic. There isn’t any big explanation for the fantastic events of the story, only some half theories advanced by the characters in the story, so if you like things all tied up in a bow you might find that part frustrating. To me, it actually made the story seem more real: in the real world, you don’t always get great explanations.
This book has a lot of structural similarities to Scott Sigler’s “Earthcore” (which I highly recommend). A small team sets out on a difficult mission under secrecy and time pressure to a place not visited before. The characters are well fleshed out with interesting motivations and interactions. There are intra-character politics and secret motives that threaten the mission. Then, weird things start happening, and before you know it everyone is running for their lives. If you like that pattern, I highly recommend both books!