A puzzle that codebreakers have yet to crack sits just outside of the CIA’s cafeteria in Langley, Virginia. Inscribed on Kryptos, a sculpture erected on the intelligence agency’s grounds in 1991, the code consists of 865 letters and four question marks punched into a curved wall of copper. Though three of its passages were successfully decoded in the 1990s, Kryptos’ fourth and final section has proven harder to solve than originally anticipated.

Now, reports John Schwartz for the New York Times, sculptor Jim Sanborn has released a new clue to the 97-character passage: “Northeast.”

The one-word hint—a decryption of letters 26 through 34—is the third and final clue Sanborn is willing to offer. The other two hints—“clock” and “Berlin,” released in 2010 and 2014, respectively—sit back-to-back at positions 64 through 69 and 70 through 74. Sanborn released the first clue about 20 years after the sculpture’s unveiling and the second on the 25th anniversary of the fall of the Berlin Wall, an event he says influenced the encryption’s development.

Why release the final clue now?

“Well, it is very close—within days—to when I actually developed that 97-character string,” Sanborn tells NPR’s Mary Louise Kelly. “The dedication ceremony is actually not until November, but obviously prior to the dedication, I had to come up with the final clue section. And that’s why I’m doing it now, basically.”

Only Sanborn and former CIA director William Webster have the solution to the encrypted message, which Sanborn developed with help from Edward Scheidt, retired chairman of the CIA’s Cryptographic Center.

At the dedication, Sanborn gave Webster two envelopes: one with the key words needed to break the code, and the other with the fully translated message, reported the Associated Press’ Robert Andrews at the time.

Perhaps unsurprisingly, the sculpture quickly captured the attention of both intelligence agency employees and the general public.

“Everyone wants to know what it says,” Sanborn told the AP in 1991.

Two men even tried to scale the sculptor’s apartment and peek through the window for clues.

“They’re out there all the time,” Sanborn added. “There are groups of dark-suited people pointing at it and getting down on their knees, trying to figure out what it says. Some take photographs. One guy copied the whole thing down with pencil and paper.”

When CIA physicist David Stein solved the puzzle’s first three passages in 1998, he called a meeting to announce his results. As Schwartz reported for the Washington Post in July 1999, some 250 people showed up to hear what the physicist, a traditionalist who at the time didn’t even have an e-mail address, had found using “pencil and paper alone.” Around the same time Stein released his findings, a computer scientist named Jim Gillogly cracked the code by creating programs that performed all of the grunt work.

In full, Kryptos’ first passage reads, “Between subtle shading and the absence of light lies the nuance of iqlusion.” (Sanborn left misspellings and extra characters to throw codebreakers off track but otherwise used classic ciphers.)

The second passage—which begins, “It was totally invisible, how’s that possible?”—directly references Webster: “It’s buried out there somewhere. X Who knows the exact location? Only WW.”

The third passage, meanwhile, references Egyptologist Howard Carter’s account of opening King Tutankhamun’s tomb: “With trembling hands I made a tiny breach in the upper left-hand corner. And then, widening the hole a little, I inserted the candle and peered in.”

New developments in the case have trickled in over the years. In 2006, Sanborn revealed that the original translation of the second passage featured a mistake in the last several words due to the omission of an extra character he’d forgotten to include. And, in 2013, the NSA announced that its codebreakers had actually solved the cipher first, in 1993.

But the fourth passage remains elusive, even with an online community of more than 2,000 people dedicated to unraveling its secrets. One of the group’s founders, Elonka Dunin, is a game designer who maintains a webpage with a trove of resources about Kryptos. She has worked with both Sanborn and Scheidt to draw out more information on the final passage. For one thing, the pair told her, the final passage uses a layered cipher.

“[Scheidt has] said our challenge is to first figure out the masking technique that was used,” Dunin told Eurogamer’s Cristian Donlan in 2014. “What that masking technique is, we don't know. It could be that he removed all the vowels from the plaintext. It could be that the plaintext was converted into binary, ones and zeroes, and then encrypted. … Then again, it’s possible that he was misdirecting us. He does work for the CIA.”

For Sanborn, the project has lasted longer and become something bigger than he originally expected. In 1998, the artist told Schwartz that he expected the first three passages to be solved within a few years and the final section within a decade. Now, at 74, he says that he’s forced to think of ways the puzzle may last after his death. He’s considering auctioning off the solution, and if it brings in a considerable sum of money, donating those funds to climate research.

Regardless of when the fourth passage is finally solved, the sculpture will retain a degree of intrigue. The 97 characters, once translated, yield a riddle, and the four passages combined comprise another puzzle. The objects surrounding the copper wave—slabs of granite, petrified wood and a pool of water—not only block sections of the text from photographs, but also contribute to the larger puzzle. Additionally, the copper is embossed with lines of Morse code.

“I would think that every artist would aspire to making an artwork that is not transient,” says Sanborn to NPR. “It’s a permanent visual, auditory, conceptual statement. And I did Kryptos with all those things in mind. … This has lived way beyond all of my expectations, you know, at 30 years in retaining a secret that it has. That’s the magic.”