From the author's website:
What waits Beneath?
As much as the story moved quickly and the characters were captivating, however, the story is somewhat incomplete. There are many holes and untied resolutions for the reader's satisfaction. This is undoubtedly answered in the sequel book, Above, but if you're like me, you are disappointed by the incomplete nature of a book made for a sequel. I would much rather have a complete, twice-thick book than two books that tell half-tales.