The CIA headquarters in Langley is home to lost of mysteries, but one in particular has captured the attention of professional code breakers and puzzle enthusiasts for the last three decades.

Kryptos, a coded sculpture created by American artist Jim Sanborn has remained unsolved ever since it was originally installed at the CIA headquarters in November of 1990. The agency’s code breakers as well as other professional puzzle solvers around the world have been struggling to unravel the secrets of the 1,800 characters carved into the 12-foot block of copper, and although they’ve succeeded in cracking three of the coded messages created by Sanborn, the fourth and final one remains unsolved. Experts have tried every technique in the book, transposition, binary, polyalphabetic substitution, even Morse code, but so far no one has been able to decipher the 97-character-long fourth part of the Kryptos Sculpture.

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