Saw it twice, amazing movie. It's nothing like a hollywood based movie. *Spoilers* You never actually see what's causing all this chaos*...barely any CGI..at least not until the end.
By the way, theres an original ending, and two alternates...the one alternate being made by Steven Spielberg and is the one you see in theaters.
Favorite horror movie, and probably one of my scariest, mainly because I went in there not knowing what was going on and thinking it was a REAL movie up until the end. I didn't sleep afterward.
If anyone wants to know the other two endings, that the director had originally intended...PM me.