Anonymous internet puzzle series (2012-14) recruiting cryptographers. Mixed Mayan numerology, prime number theory, and physical drops at GPS coordinates. Still unsolved at the deepest level.
Cicada 3301 are three sets of puzzles posted under the name "3301" online between 2012 and 2014. The first puzzle started on January 4, 2012, on 4chan, and ran for nearly a month. A second round began on January 4, 2013, and a third round was posted on Twitter on January 4, 2014. The third puzzle remains unsolved. The stated intent was to recruit "intelligent individuals" by presenting a series of puzzles to be solved; no new puzzles were published on January 4, 2015. A new clue was posted on Twitter on January 5, 2016. Cicada 3301 posted their last verified OpenPGP-signed message in April 2017, denying the validity of any unsigned puzzle.