3. The Beale Ciphers
This one is particularly exciting, as it sounds like something out of an Uncharted game.
The story goes that treasure hunter Thomas Jefferson Beale visited an innkeeper named Robert Morriss in 1820. This was only the first encounter the pair would have, as two years later Thomas would return, leaving a mysterious box in Morriss' care.
23 years later, the innkeeper decided to open the box, and in it, he found three coded messages. These supposedly would lead to a vault of buried treasure, estimated to be worth over $43 million in today's value.
Morriss ended up passing the codes onto an unnamed friend after his death, who only succeeded in cracking one of them after 20 years of work. Eventually, he published the notes into a pamphlet, but even then, no one has solved the mystery.
It seems that no one will be finding the gold at the end of this rainbow or finding out if the treasure even exists. There are countless theories that the entire thing is one big practical joke or a simple hoax. Still, it would be nice to know either way.