It's far from the last great unsolved mystery, though, as claimed in the title: I would definitely put (i) deciphering the last part of Kryptos (http://en.wikipedia.org/wiki/Kryptos), (ii) deciphering the Voynich Manuscript (http://en.wikipedia.org/wiki/Voynich_manuscript), and, if you insist on adding treasure to the equation (iii) the Beale Ciphers (http://en.wikipedia.org/wiki/Beale_ciphers) on any list of unsolved mysteries that fuel the imagination.