In 1988, artist James Sanborn was commissioned to create an outdoor sculpture to adorn the CIA's facility in Langley, Virginia. So he created Kryptos, a 10-foot high scroll of copper filled with letters. Its 865 characters contain, the artist asserts, a coded message. But even the best CIA cryptologists have been unable to crack all of it. One of the four sections remains a complete mystery. At the link, you can read about Sanborn's extensive study of cryptology while planning the sculpture and the passion that it has inspired among devoted codebreakers.
Link via Instapundit