So, you think you're good at decyphering codes, eh? Try this one on for size:
Kryptos is a sculpture built outside CIA headquarters in Langley, Virginia, and even the CIA's own cryptologists can't crack it. The sculpture, made by artist James Sanborn and encoded by retired CIA cryptologist Ed Scheidt, has been driving agents insane for nearly two decades now. I saw this on Wired today and thought it was definitely worthy of sharing with you folks

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The first three sections (K1, K2, and K3) have all been solved already, and it took seven years for the agency to solve them. K4, the final segment of 97 characters, remains a mystery.
Here's some info regarding the puzzle (page 3 of a 4-page article about Kryptos).