The Kryptos sculpture is located at the CIA headquarters in Virginia, and is made up of four heavily encrypted sections. Three of them have been decoded, but the fourth remains unsolved from its 1990 installation, and is one of the most fiercely chased quests of modern codebreaking. All four sections are meant to then form a riddle, which has to be solved. I don't think there's any prize, apart from being a BAMF for breaking codes. The three solved sections are part treasure map instructions, part poem, part discussion. When it's finally solved, I can't wait to see where it leads.