Although, or better to say because, the book is rather short (I needed about 4 hours to read it) it is intense and grotesque in many aspects.
First, the formal layout of the book is three chapters, written in the first, second and third person perspective. This produces in particular for the two first two chapters a personal almost intimidating experience. This is emphasized by the rather erratic language, which are more an assembly of half-sentences and second thoughts than well written prose, but it serves its purpose to enhance the claustrophobic, dooms-day feeling of the main characters.
The central part is the last chapter (I regard the first two chapter as a prologue to it) describing the voyage from the surface to the deepest level of the underground. It feels like a modern version of the Dante's Inferno. Vandermeer describes that which the progress in the underground humanity is more and more withdrawn. First it is only reflected in the behavior of people living there in despair. Then even their appearance alters (like the reappearance of the main character of the first chapter). Further down the underground is populated with creatures which only remaining humane character treat is suffering because they recognized the agony to live in that place and the awareness of their own doomed and flawed existence. At the end even that is gone and what remains is a chaotic dog-eats-dog world.
I rarely encountered a book which provoke so much emotion while reading and long after that. The book defies any classification into SF or Mystery and its use of first and second person narrative makes it so distinct to other who tried a similar approach.