Kryptos

Kryptos is an encrypted sculpture by American artist Jim Sanborn located on the grounds of the Central Intelligence Agency (CIA) in Langley, Virginia. Since its dedication on November 3, 1990, there has been much speculation about the meaning of the encrypted messages it bears. Of the four messages, three have been solved, with the fourth remaining one of the most famous unsolved codes in the world. The sculpture continues to provide a diversion for cryptanalysts, both amateur and professional, who are attempting to decrypt the final section.
The main sculpture is located in the northwest corner of the New Headquarters Building courtyard, outside of the Agency cafeteria. The sculpture comprises four large copper plates with other elements made of red and green granite, white quartz, and petrified wood.
The name Kryptos comes from the Greek word for "hidden", and the theme of the sculpture is "intelligence gathering." The most prominent feature is a large vertical S-shaped copper screen resembling a scroll, or piece of paper emerging from a computer printer, covered with characters constituting encrypted text. The characters consist of the 26 letters of the standard Latin alphabet and question marks cut out of the copper. The main sculpture contains four separate enigmatic messages, three of which have been solved.
At the same time as the main sculpture was installed, sculptor Jim Sanborn also placed several other pieces around CIA grounds, such as several large granite slabs with sandwiched copper sheets outside the entrance to the New Headquarters Building. Several morse code messages are engraved in the copper, and one of the slabs has an engraved compass rose and a lodestone. Other elements of Sanborn's installation include a landscaped area, a duck pond, a reflecting pool, and several other seemingly unmarked slabs.
The cost of the sculpture was $250,000
The Kryptos Project website is 
More information - including transcripts of the text - on the Wikipedia page .
The Mengenlehreuhr is linked to the decipherment of the fourth section of the Kryptos sculpture.
There is various other coverage of the sculpture.
Category:Uncracked codes and ciphers
Category:20th century