The Kryptos puzzle sculpture at the Central Intelligence Agency in Langley, Virginia.
Kryptos is a sculpture, created by artist Jim Sanborn in 1990, that contains secret cryptographic codes. Three of the four encrypted puzzles were solved by 1999, but the last part has proven maddeningly difficult to decipher, so the artist has given the answer to six letters in the last passage.
Mr. Sanborn made the puzzle more difficult by deliberately misspelling some of the words. Also, the last section is short, which makes it more difficult to decode.
"Anybody holding a secret has a position of power, even if it’s a trivial secret," says the artist.
Lots of information on this puzzle at Elonka’s Kryptos page>>
Clues to Stubborn Secret in C.I.A.’s Backyard, The New York Times>> More on Kryptos>>