The Kryptos sculpture (ded. October 1990) in the CIA's courtyard (doesn't 'The CIA's Courtyard' sound like a rejected Clancey novel title?) has yet to be hacked. The last I heard, a computer scientist, Jim Gillogly nailed the first sections, but the sculptor used a shifting cypher that keeps potential crackers guessing (or more likely, brute-forcing keys). If you'd like to try it yourself, a transcription of the sculpture's code is here. It'll be tough. It was designed to be unsolvable.