Pirate Legend/Undiscovered Content and Clues DISCUSSION

I feel like we need a place to talk about all of the mysteries in Sea of Thieves, and what clues we have as to what they are! There is clearly a lot of stuff in the game that will not be discovered until people start becoming pirate legends. Nevertheless, there are tons of clues around the world such as mysterious underwater caves that seem to have an unknown function, and books with riddles talking about pirate legends and where they may dwell. Let's talk about what we know, and what we can do to discover more clues!