It's not that only the future is unknown.
Our history is full of mystry as well.
From 'The City of Atlantis' to 'The voynich Manuscript' or from 'the Mystery of DB Cooper' to 'the unsolved murder of JFK', there are numerous events or personas or things that remain mysterious till date.
You must have known many such mysteries already but, sadly you could only settle with speculations about them. That must have been funny, intriguing and mind scratching. Wasn't that?