The Descent

If you want to watch the scariest movie ever made, go watch The Descent. A british horror movie about a group of girls who go caving in the Appalachian mountains. After getting trapped in the system (with claustrophobia and desperation setting in), they start to realize they *might* not be alone.

There's one scene in particular that's the scariest thing I've seen in movies. I've had grown men scream like little girls while watching it. Awesome stuff. Watch it in the dark, if you dare.