If you have viewed the front page of this site, you can see that this one is #3 on my all time favorites list.  That is certainly not an accident.  I have yet to meet a horror fan that does not speak highly of this one.  The film was directed by Neil Marshall, who gained the respect of many with his work on this one.  I see on IMDB that The Descent 2 is in production and fear that the sequel will be a letdown.  The reason being that the cast appears to be much the same.  Since most of them appeared to die in the first, I am not sure how this will work.
First and foremost, the acting in this film is excellent.  The film follows a group of women exploring a large cave.  The only problem is, that this cave is not mapped out and they have been led to a point where they cannot find an exit.  The women have a believabilty and a clear bond.  The interactions are so genuine that you almost forget that they are staged.  The characters are very well developed via the script and you begin to care what happens to them.
Next, the suspense of the film is classic.  The film shows you just enough to know that something really bad is lurking in the cave, but it cannot be readily identified.  When I did see the "monsters" in this movie, I was blown away.  So simplistic on one hand, but enough forethought to show how they have evolved in the environment.
The gore scenes are fantastic.  The blood flows freely and the scenes are very realistic.  Mr. Marshall also showed that he is very adept in directing this type of movie.  The kill scenes certainly have an absolute brutality to them , but remain very realistic.  When another director would pull the camera away, he instead chooses to show the graphic result of these attacks.
Not much more to say as I do not wish to spoil the surprises for anyone.  I suspect that this one will saty at #3 on my list for a long. long time.  I feel that any film that stays in my top 5 deserves a 9 in the rating scale!