I guess I'll give a little synopsis for ya. A group of thrill-seeking young women go on a caving expedition and end up being stalked by these weird alien-looking creatures (the best description I can think of at the moment, sorry). The first half of the movie is really about them going through the caves and actually keeps you really engaged. This is a movie where you truly don't see what's coming next.
Give this a shot. It's one of the good ones.