To unwitting observers, the 20-foot-long copper sculpture appears to portray a long scroll adorned with thousands of seemingly random and meaningless letters.
But these see-through characters harbour an encrypted translation that has eluded the CIA employees who pass it daily – and thousands elsewhere seeking to uncover it – for three decades.
And there is more to Kronos than the copper scroll. Snippets of Morse code are littered throughout the overall creation, which also involves a pillar of petrified wood, a reflecting pool, and slabs of granite.
The scroll itself is divided into four passages. So far, three of these have been cracked.
The first three of these were quietly solved by National Security Agency (NSA) officials in 1992, a Freedom of Information request showed years later. This first part of the passage reads:
“BETWEEN SUBTLE SHADING AND THE ABSENCE OF LIGHT LIES THE NUANCE OF IQLUSION.”
Sanborn – who says the misspelling of "illusion" was deliberate – later released two clues, in 2010 and 2014.
The second passage was revealed to state the coordinates of the CIA's Langley headquarters, and references "WW", taken to be William Webster – who led the agency when the sculpture was unveiled. It says:
“DOES LANGLEY KNOW ABOUT THIS? THEY SHOULD: IT’S BURIED OUT THERE SOMEWHERE. X WHO KNOWS THE EXACT LOCATION? ONLY WW.”
The third passage parrots lines from Howard Carter's account of discovering Tutankhamun's tomb, again with deliberate spelling mistakes:
“SLOWLY, DESPARATLY SLOWLY, THE REMAINS OF PASSAGE DEBRIS THAT ENCUMBERED THE LOWER PART OF THE DOORWAY WAS REMOVED. 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. THE HOT AIR ESCAPING FROM THE CHAMBER CAUSED THE FLAME TO FLICKER, BUT PRESENTLY DETAILS OF THE ROOM WITHIN EMERGED FROM THE MIST. X CAN YOU SEE ANYTHING? Q”
Now, with the final passage having eluded the world for decades, Sanborn has given a final clue.
It comes in the form of a one-word cipher: NORTHEAST.
However, even if this clue proves enough for the codebreaking community to solve the mystery of the final passage, there is a deeper layer to Kryptos – the four passages also contain a riddle.
“There will be yet another mystery that the four passages together have a meaning that’s greater than their individual pieces and there’s something more to figure out,” Dr Craig Bauer, a York College of Pennsylvania maths professor – formerly of the NSA’s Centre for Cryptologic History – told The New York Times.
"This is the sculpture that keeps on giving," he added.
For those who think they might have cracked it, you can email Sanborn via his website, in a process costing $50.
This might seem steep, but there is also a cost to Sanborn for managing to mystify the world for three decades, with the 74-year-old reportedly no stranger to disgruntled codebreakers showing up at his home over the years.
According to The Times, Sanborn has decided that if his secret outlives him, the secret will be put up for auction, with the proceeds to be used to fund climate science – a decision, he said, "seemed like a no-brainer".