In early 2012, a 4chan user posted a steganograph (a way to conceal data within code, text, images...) that said someone or something called Cicada 3301 was recruiting intelligent people. It's your classic White Rabbit trope, but if you could make it beyond the straight-forward Caesar cipher in the text, you would begin to unlock deeper, more technical layers of the puzzle and eventually find yourself...???
And that's the question. More puzzles followed and before long, like Slender Man before it, Cicada 3301 was a 4chan celebrity.
Some claim that they solved the seemingly unsolvable riddles and received e-mails from the entity, but beyond that it isn't clear what winners have received. Others posit that Cicada 3301 is a front for information security firms or even government agencies. The ability to crack complex codes is in high demand.
Or maybe it's not one person or group at all. Maybe it started out as a single poster with a puzzle and then others added their own under the same name. The reward being little more than knowing you could solve a complex puzzle.