The theory, which came about in 2013, originates from Reddit user atomicbolt. In a nutshell, the theory claims that Sandy drowned at the start of the film, and everything was just a fantasy.
Yeah, it's dark, right? Turns out Sarah is just as confused as you are about the whole thing.
But not everyone was buying the theory, with a few commenters pointing out some flaws.
But some were totally into it.
Maybe this is just another ~mystery~ the Scooby Gang will have to solve.