What is Cicada 3301?

Sleepily – it was late, and he had work in the morning – Eriksson thought he’d try his luck decoding the message from "3301”. After only a few minutes work he’d got somewhere: a reference to "Tiberius Claudius Caesar” and a line of meaningless letters. Joel deduced it might be an embedded "Caesar cipher” – an encryption technique named after Julius Caesar, who used it in private correspondence. It replaces characters by a letter a certain number of positions down the alphabet. As Claudius was the fourth emperor, it suggested "four” might be important – and lo, within minutes, Eriksson found another web address buried in the image’s code.

Feeling satisfied, he clicked the link.

It was a picture of a duck with the message: "Woops! Just decoys this way. Looks like you can’t guess how to get the message out.”

"If something is too easy or too routine, I quickly lose interest,” says Eriksson. "But it seemed like the challenge was a bit harder than a Caesar cipher after all. I was hooked.”

Eriksson didn’t realise it then, but he was embarking on one of the internet’s most enduring puzzles; a scavenger hunt that has led thousands of competitors across the web, down telephone lines, out to several physical locations around the globe, and into unchartered areas of the "darknet”. So far, the hunt has required a knowledge of number theory, philosophy and classical music. An interest in both cyberpunk literature and the Victorian occult has also come in handy as has an understanding of Mayan numerology.

A gripping tale of a mysterious puzzle posted online. By whom? And for those who solve the puzzles, what awaits?

Let's hope this isn't a promotion for the next Dan Brown novel.