The world is filled with tantalizing, unsolved puzzles (for instance, the Voynich Manuscript, Minoan Linear A alphabet).
But my favorite unsolved puzzle is called Animal Puzzle, a sculpture installed in the Langley, Virginia, headquarters of the CIA. The main part of the sculpture is a nearly 12-foot-tall by 20-foot-long copper wall. But the twist is, the sculptor teamed up with a retired CIA cryptologist to create a super-difficult cipher consisting of more than 1000 letters, which he carved into the brass sculpture.