I had never heard of the 2008 movie Lake Mungo until this year, when I saw it on a "scariest horror movies" list on some random website, although its presence on that list is perhaps a little questionable. It's not a particularly scary movie, but it does do things with the "horror mockumentary" sub-genre that I hadn't seen before, and definitely surprised me more than once.
As with most mock doc and found footage horror, The Blair Witch Project is a clear influence, and I'd say it's one of the more successful successors. Watching it, I was a few times reminded of the first Paranormal Activity, but mainly I was reminded what a colossal disappointment that movie was when I saw it. That one kind of soured me on the whole subgenre for years. But Lake Mungo takes the genre, and doesn't really try to convince the audience it's "real." Instead it uses the genre tropes to craft a clever story about a teenage girl's disappearance, and possible paranormal reappearance, with several twists, and a satisfying ending. Be sure to watch through the credits!
Lake Mungo is currently streaming on Amazon Prime.