Some sciencey people go down in a cave for some reason and there's some sort of creature that's killing them.
I usually give a more detailed plot, but I wasn't paying too much attention to this. Overall, it was dull and the only time you'll be really paying attention is during the action scenes, which the director did wonderful on.
The acting is alright, but the characters are so dull and forgettable they blend in your mind. You'll forget who lived and who died for 2 reasons: 1. The kills are boring 2. The characters are boring.
The ending might have shocked me more if I knew who was who.
So you're looking for a creatures-in-cave movie? Check out The Descent instead.