Kryptos is a sculpture outside of the CIA headquarters in Virginia, America. It consists of four copper panels each of which is covered in code. Letters which spell out nonsense, until you figure out the encryption. Each of the four panels is encrypted in a different way. Since its installation in 1990 three of the four panels have been decoded. Part remains unsolved after these 20 years despite the efforts put in by the employees of the CIA and thousands of cryptanalysts. This fourth and final panel of Kryptos is now one of the most famous unsolved codes in the world.
The sculpture was a $250,000 piece by Jim Sanborn. Around the CIA site he placed many other, smaller but similar copper panels with messages in morse code engraved in their surfaces; for Kryptos another level was required. A message was created, split into four panels then each encoded with the help of the chief of the CIA Cryptographic Center. Each panel using a more complex combination of complex ciphers.
The fourth panel has remained elusive for a reason. It is greatly more complex than the others, to solve it, you both need to find the keys for it, what ciphers were used, and you also need to use the answers from the previous three panels as well. Without the first three answers perfect, the fourth remains impossible.