Reddit top answers: (https://www.reddit.com/r/AskReddit/comments/5fpuur/what_is_the_greatest_unsolved_mystery_of_all_time/)
A mystery that really intrigues me is the the Indus Valley Civilization. They were large well planned cities on the banks of the river Indus. They had an advanced sanitation system and their people were great designers/builders.
Eventually the people started to abandon the cities or maybe they perished. The cities were excavated years later by archaeologists. No one has been able to exactly pin-point the cause of the destruction of the cities (there are multiple theories though). They had an entire script that no one has been able to translate.
The archaeologists have unearthed idols, buildings, utensils, money (coins). It's just interesting how they just ceased to exist for so many years.
The guy could be dead by now and we're still thinking he's out there.
The Isabella Stewart Gardener Museum (Boston) theft of 13 oil paintings worth $500 million, March 18, 1990 - still unsolved.
Dude steals money off a government plane, asks for a parachute, jumps from the plane never to be seen again...
IMO the [Battle of Los Angeles](https://m.youtube.com/watch?v=5m7736RMBEg)
Long story short an object was spotted hovering slowly over LA during WWII. The entire Los Angeles anti aircraft battery engaged it for like an hour thinking it was a Japanese aircraft but couldn't damage it. Then after a while it just disappeared as suddenly as it had appeared