2. Cicada 3301, 2012
A series of highly-complex online puzzles set up by a mysterious company in hopes of recruiting "highly intelligent individuals" from the public. Some speculate it’s the work of the NSA, MI6, or CIA, and only a few people have gotten far in the process of solving it. Originally found at ground zero for Internet folklore, 4Chan, Cicada 3301 has been called “the most elaborate and mysterious puzzle of the Internet age.”
Only a few people have solved it, notably a 34-year-old cryptosecurity researcher and developer from Sweden named Joel Eriksson, who believes a private organization of hackers is behind the series of puzzles.