For 3 consecutive years (2012, 2013, 2014, … not 2015, though) a mysterious organization called Cicada 3301 posted extremely hard and elaborate puzzles online, sometimes with real-world clues appearing all over the world. Their intent is to find highly intelligent individuals, but not much more is known. Is the intelligence community behind this? Headhunters? No-one knows. A fascinating digital mystery. More info if you continue reading.