For nine years it has challenged all who pass to reveal its message.
"I am the only person who knows the answer," says artist Jim Sanborn, a sculptor with a flare for codes. Sanborn spent two years carving 1,706 characters into the CIA-work, named Kryptos, the Greek word for hidden. He's rooting someone will crack it.
"I didn't necessarily want this to be a mystery forever. I mean, that wasn't the reason I did it," the sculptor says.
Some of it has been cracked. David Stein is a physicist for the CIA - a scientist not a spook.
"I didn't know very much about codes at all. I had done simple mono-alphabetic substitutions as a child with my mother, actually," Stein says.
Even though he's not a professional code-breaker, the methodical Stein, approaching the task like a scientific problem, has solved all but the last 97 letters. He does it all without electronic aids of any kind.
"For me, that's the enjoyment of it," Stein says.
Across the country in California, Jim Gillogly, who is not CIA and hasn't even seen Kryptos in person, is also within 97 letters of a solution.
"On the last part, the 97 characters, I've spent most of the evenings for a month and haven't made any progress on it," Gillogly says.
But Gillogly's decoding, unlike Stein's, is high speed, high tech.
A computer scientist, he writes programs that subject the code to billions of combinations in the search for answers. Thus the Kryptos mystery is now something of a contest between man and computer, each code-breaker championing his own methods.
"The code was designed to be broken with paper and pencil," says CIA physicist Stein.
But Gillogly argues: "The whole idea of solving a cryptogram is finding a solution. And if the best tool to do that is a computer, then you might as well use it."
But let's not get ahead of ourselves. Those last 97 letters, when deciphered, may still not yield the final answer.
"Once it is deciphered, it is still going to retain a mysterious aspect. It is not a simple ending," says sculptor Sanborn.
Like any good spy story, it seems Kryptos won't yield a clear-cut answer, but will end in the shadows of enigma and intrigue.
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