The Descent. First of all, it passes the golden rule of a good horror film. If you walk out of the theater and call the characters by their names, you have made a good horror film, and this film performs in spades, as once the creatures come out and start killing the group of cave divers, characters start to show their true colours.
The story follows a group of female cave divers as they try to help one of their own, Sarah, get over the traumatic death of her daughter and husband by distracting her with a cave dive. Little known to the rest of the group, Juno, the wanna-be-gung-ho-leader took the group into an unmapped cave system. After a cave in, the girls must go deeper into the uncharted territory with the hopes of finding an exit.