The reviews for this were adulatory; it's almost as if this is the Citizen Kane of horror movies. I don't quite buy that, sorry. It's not bad, though, just not for the squeamish.
As usual, there are lessons: if your yearly thrilling adventure vacation with your girlfriends ends in bloody tragedy one year, you should probably skip it next year. But noooo, nobody follows that commonsense advice here. Instead they go spelunking, get incredibly lost, and meet up with some guys that make Gollum look like George Clooney. A bad time is had by all. There's a lot of noise, shaky camerawork, and fast cutting, which usually indicates the special effects budget wasn't big enough.
No spoilers, but if you're at all familiar with the genre, you'll know it does not end with all the people getting out of the cave, brushing off the cave dust, congratulating each other on how their intrepid teamwork, quick thinking, and courage rescued them from a close scrape.