In 1999, The Blair Witch Project created an entirely new genre of movie that melded a documentary with fictional story telling. The premise was that some indeterminate time after a horrific event or disappearance authorities found video footage of the event and pieced together the documentary from it.
When Blair Witch came out, there was quite a bit of drama about whether it was real or if the Blair Witch was actually a phenomenon. They did an amazing job of milking this new public naivety to the hilt and made millions from what was a small independent project. Like all good things, Hollywood abused it and soon there were countless found footage films coming out of the woodwork.