9. The Vela Incident
On 22 September 1979, the US satellite Vela 6911 noticed an unusual bright 'double flash' off the coast of South Africa in the confluence of the South Atlantic and Indian Ocean.
To this day, no one has ever confirmed what the flash was, but whatever caused it, it was big. The flash was accompanied by seismographic and atmospheric readings that suggested something huge kicked off. Theories include an underwater earthquake, a freak weather event or a meteorite strike.
The scientific consensus has focused on one possibility, however. It was probably a nuclear bomb.
If it was a nuke being tested, there are two likely culprits - Israel, and South Africa. These two had their own nuclear weapons programs and, if they did set one off in 1979, they breached every nuclear non-proliferation treaty in effect and got away with it.
The very real possibility exists that someone detonated a nuke in secret and no one ever found out who. Why would they do it? Who knew about it? And if it wasn't somebody sneakily joining the nuclear club... what the hell was it?