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(CNN) In the middle of CIA headquarters, there sits a sculpture that contains a secret code that has stumped top cryptologists for decades.
In the late 1980s, artist Jim Sanborn was commissioned to create a sculpture to be displayed at CIA headquarters in Langley, Virginia. Knowing some of the world's top intelligence officials would see the piece practically every day, Sanborn made a work of art that is, in a word, puzzling.
Unveiled on November 3, 1990, it's called Kryptos, and it contains a cryptographic challenge. Surely, someone would crack the code in just a couple of weeks, Sanborn thought.
But no one did. And, today, Kryptos remains one of the world's most famous unsolved mysteries.
"I didn't think it would go on this long -- thirty years -- without being deciphered," Sanborn says.
Kryptos sits in a courtyard outside the CIA headquarters. A curvy, copper screen measuring 12 feet tall and 20 feet wide, the sculpture is packed with letters. "I cut with jigsaws, by hand, almost 2,000 letters," Sanborn says.
The text that covers the sculpture looks like gibberish to the untrained eye, but Kryptos contains four distinct, encoded messages that together form a riddle, according to Sanborn.
Sanborn had no experience in the art of writing code before he created Kryptos. And he wasn't particularly good at math.
"So I had to turn to somebody. Was it going to be the Soviets? Was it going to be Mossad? Was it going to be some other intelligence agency?" he cracks. "I pretty much crossed them all off because it probably wouldn't end well."
The artist ultimately sought guidance from Edward Scheidt, an expert in cryptology and encryption who had been chairman of the CIA's Cryptographic Center before he retired.
We met more or less in secret, and he educated me on the subject of code, modern codes, contemporary coding systems -- at least contemporary in '88," Sanborn says.
Using what he learned from Scheidt, Sanborn composed encoded text messages to embed within Kryptos using various techniques, including patterns and matrixes.
"I wanted it to reveal itself like peeling layers off an onion," he says of the mystery carved into Kryptos.
Three of the encoded messages have been deciphered over the years by members of the NSA, CIA and the general public.
But more of the onion needs to be peeled away.
The first message reads: "Between subtle shading and the absence of light lies the nuance of iqlusion."
"Iqlusion" isn't a typo. Sanborn intentionally misspelled the word "illusion." It was his way of throwing people off.
The second message, which also contains a misspelling, shares latitude and longitude coordinates for the CIA, hints something is buried there and references "WW."
The initials are a nod to William Webster. He was the head of the CIA in 1990 when Kryptos was installed.
The third message borrows from archaeologist Howard Carter's writings about opening the door to King Tut's tomb.
So, what does all this mean?
That's still a mystery. We'll be a step closer when someone finally cracks the code behind the fourth message.
There are many who think they have, but according to Sanborn, no one has yet.
"I have one individual who contacts me once a week at exactly the same second -- I think it's Tuesdays at 8:23 in the morning -- with a decrypt, and this person has been doing this for two-and-a-half years," Sanborn says.
One cryptologist has been trying to solve Kryptos for 20 years
Sanborn has gotten to know some of the people working to solve Kryptos. He admires the efforts of Elonka Dunin and says she "probably knows more about Kryptos than I know."
Dunin, a cryptologist and video game developer, first heard of Kryptos in late 2000. She loves to solve puzzles, but she knew she couldn't figure out Kryptos alone.
She created a website dedicated to the code. It's grown into a network of people committed to a common goal.
"Our Kryptos community is made up of people all around the world. We have thousands of people that are interested in Kryptos -- either cracking it, or helping to see it cracked," she says. "Some of them are professional code breakers. Some of them are students."
Dunin organizes events at which code breakers can grill Sanborn as well as Scheidt in person. They usually meet every couple of years at a restaurant, or at Sanborn's Maryland studio.
Sanborn appreciates the group's enthusiasm, especially given that there is no reward for solving Kryptos beyond bragging rights.
That said, he isn't making it any easier for anyone.
Over the years, Sanborn has publicly shared three one-word hints from the elusive fourth passage -- "clock," "Berlin," and, most recently, "northeast" -- to help the process along.
But that's it.
Dunin can't predict exactly when it will happen. But she does believe Kryptos will be solved one day. "I would feel, personally, an immense feeling of relief. It would be off my plate," Dunin says.
And she doesn't care who figures it out at this point. "It doesn't have to be me," she stresses. "I want to see it solved."
Does Sanborn, who keeps the solution to Kryptos hidden away in a safe deposit box, think it will be solved? Does he even want it to be solved?
"I wouldn't be distraught if it ended tomorrow," Sanborn muses. "I'd maybe be relieved."