Outside the Central Intelligence Agency (CIA) headquarters in Langley, Virginia, there is a 3.7-meter (12-foot) tall copper scroll covered in seemingly nonsensical letters, known as Kryptos.

The puzzle, designed by Jim Sanborn, has gone unsolved for over three decades – despite efforts from internet sleuths, the NSA, and the CIA itself, who get to see the sculpture every day on their commute. Three of four passages hidden on the puzzle have been decrypted, but one remains elusive.

Here's what we know/don't know so far.

Passage one

This passage turned out to use a Vigenère cipher, a way of encrypting text using two Caesar ciphers – which is a simple method of encryption that shifts letters earlier or later in the alphabet – interwoven together. 

The keywords used for this cipher were "Kryptos" and "Palimpsest". First, the word Kryptos is placed at the top of a grid (see video below) with Palimpsest beneath the K going downwards. After Kryptos, the alphabet is continued in alphabetical order, skipping any of the letters within the word Kryptos.

This is continued on the subsequent lines, starting from the letter in the word Palimpsest.

Then to decrypt the first letter of the passage – in this case, an E – you scroll across the first line until you find an E, then go up to the top to reveal what it should be – in this case, the letter B. The same principle continues for the rest of the text, eventually giving you the message (with a deliberate typo):

BETWEEN SUBTLE SHADING AND THE ABSENCE OF LIGHT LIES THE NUANCE OF IQLUSION

Passage two

This passage used the same encryption method as the first passage but with the keywords "Kryptos" and "Abscissa". When decoded, it translated to text and coordinates:

IT WAS TOTALLY INVISIBLE HOWS THAT POSSIBLE ? THEY USED THE EARTHS MAGNETIC FIELD X THE INFORMATION WAS GATHERED AND TRANSMITTED UNDERGRUUND TO AN UNKNOWN LOCATION X DOES LANGLEY KNOW ABOUT THIS ? THEY SHOULD ITS BURIED OUT THERE SOMEWHERE X WHO KNOWS THE EXACT LOCATION ? ONLY WW THIS WAS HIS LAST MESSAGE X THIRTY EIGHT DEGREES FIFTY SEVEN MINUTES SIX POINT FIVE SECONDS NORTH SEVENTY SEVEN DEGREES EIGHT MINUTES FORTY FOUR SECONDS WEST X LAYER TWO

Passage three:

Passage 3 was solved using two methods of transposition (a good explanation of which can be found here). The final message was:

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 ?

Somewhat pleasingly, the message is a slightly altered relaying of Howard Carter's account of opening Tutankhamun's tomb in 1922.

Passage four:

The unsolved passage reads:

NGHIJLMNQUVWXZKRYPTOSABCDEFGHIJL
OHIJLMNQUVWXZKRYPTOSABCDEFGHIJL
PIJLMNQUVWXZKRYPTOSABCDEFGHIJLM
QJLMNQUVWXZKRYPTOSABCDEFGHIJLMN
RLMNQUVWXZKRYPTOSABCDEFGHIJLMNQ
SMNQUVWXZKRYPTOSABCDEFGHIJLMNQU
TNQUVWXZKRYPTOSABCDEFGHIJLMNQUV
UQUVWXZKRYPTOSABCDEFGHIJLMNQUVW
VUVWXZKRYPTOSABCDEFGHIJLMNQUVWX
WVWXZKRYPTOSABCDEFGHIJLMNQUVWXZ
XWXZKRYPTOSABCDEFGHIJLMNQUVWXZK
YXZKRYPTOSABCDEFGHIJLMNQUVWXZKR
ZZKRYPTOSABCDEFGHIJLMNQUVWXZKRY
  ABCDEFGHIJKLMNOPQRSTUVWXYZABCD

So far, the message remains hidden, despite the creator revealing several clues. The first clue, revealed in 2006, was that the first three passages were clues for the final passage. Four years later, he revealed that "NYPVTT", the 64th–⁠69th letters in passage four, becomes "BERLIN" when decrypted, before going on to reveal that "MZFPK" (the 70th–⁠74th letters) means "CLOCK".

Still unsolved, he revealed that people should "look into that clock". Still too fiendish, he has gone on to reveal that the letters in positions 22–25 read "FLRV", as well as a one-word ciphertext revealing "QQPRNGKSS" is the word "NORTHEAST".

The fourth passage, once solved, will reveal yet another riddle to be solved. Should nobody be able to crack the code before his death, Sanborn has revealed he will sell the solution at auction, donating the money to climate science. Given that it's been 30 years already, you might want to save up for the auction.