Cicada 3301 is an unusual internet mystery. It leads people on a wild goose chase around the world, with physical locations, as well as many different internet leads, some dead ends, some leading places. No one really know what it is, or if it is an actual thing.
Some links with more in depth information about past ones:
http://www.cbsnews.com/news/cicada-3301-code-breaking-scaven.../
http://en.wikipedia.org/wiki/Cicada_3301
http://www.telegraph.co.uk/technology/internet/10468112/The-...
This year:
http://uncovering-cicada.wikia.com/wiki/CICADA_3301_2014_PUZ... - Wiki page about what has been uncovered this year
https://twitter.com/1231507051321 - This year, it was apparently announced on this twitter account
Previously on hubski. I guess the prevailing theory is that it's Anonymous recruiting. Or something.