Teens Die in Underground Party in 2000's Film

by Robin
(Texas, USA)

I watched this 2005 or 2006 horror movie a few years ago that greatly disturbed me. In it was a popular teen actress whose name I can't recall.

Four teens (two boys, two girls) go to some kind of underground shelter to party, and an outcast girl locks them in and doesn't tell anyone she has the key.

It starts out as a fun party but eventually someone is killed and eaten, and there is some sort of pit that makes people go crazy.

In the end, the girl with the key is the only one alive. She goes back to town and tells everyone that a crazy killer did it.

Any thoughts?