In January 2012 this image was posted to 4chan's "random" board.
Joel Eriksson, a 34-year-old computer analyst from Uppsala in Sweden, saw the message.
He knew it was an example of digital steganography: a way of hiding an encoded message in an image. A sender might, for example, start with an innocuous image file and adjust the color of every 100th pixel to correspond to a letter in the alphabet.
Eriksson tried decoding 3301's message. After only a few minutes work he'd got somewhere: a reference to "Tiberius Claudius Caesar" and a line of meaningless letters.
He worked out it was a "Caesar cypher", as used by Julius Caesar in private correspondence. From that, he found a new web address.
It led to this picture. He ran it through a decryption programme, and found another link to a Reddit messageboard. There were encrypted links to books on it, along with lines and dots.
They were Mayan numbers. And they led to another cipher.
The clues got more and more complex, with increasingly sophisticated encryption techniques required. More and more 4chan users joined the hunt. And this logo began to appear more and more.
The increasingly complex riddles were communicated in a number of different ways: music, on the "darknet", digital images, Linux CDs, and physical posters.
They have referenced (among others) William Gibson, William Blake, Aleister Crowley, Anglo-Saxon runes, and John William Waterhouse's The Lady of Shalott.
And have appeared all over the USA, in Russia, Japan, France, South Korea and Poland, prompting speculation the group is large and well-funded.
So who are they? There have been a number of different theories:
This anonymous posting on Pastebin, allegedly from a British military officer, says it's a group of "military officers, diplomats, and academics who were dissatisfied with the direction of the world". However, it's likely a fake.
It could, of course, be a recruitment drive, like the one launched by British intelligence services last year. It could also be a private company looking for cryptologists: a useful skill in the world of online shopping in particular.
Right on schedule, the puzzles reappeared on January 5th this year, on this little-followed Twitter account.
January 6 is the Christian feast day known as Epiphany.
More research through Outguess has revealed a link to a treatise by Ralph Waldo Emerson.
Running an excerpt through a cipher reveals this collage from William Blake's Marriage of Heaven and Hell.
If you want to monitor the progress of Cicada 3301's solvers check this wiki.
Will this be the year we finally discover the truth behind the mystery?