The Philadelphia Experiment sits alongside the Roswell Incident and the New World Order in the list of things the US government claims aren't really things, but that internet conspiracy theorists with low-grade camera equipment absolutely insist are.
Here, the military is claimed to have rendered one of its ships temporarily invisible to enemies during a top secret mission in October 1943. They achieved this, so it is said, by travelling about 10 minutes back in time.
This one, to be fair, does seem slightly more plausible than most. Going a few minutes into the past by messing around with electromagnetic and gravitational forces beneath the seabed is a lot more believable than the concept of a time machine in which you literally punch in your intended destination like a microwave.
But authorities have always denied that such an experiment took place at all, and the science behind it is sketchy at best. As with most of the stories on this list, it's almost completely unfalsifiable - you just have to decide whether to believe the experts or the kind of people who dress their heads in tin foil.