6. A858 Code
In July 2011, in a series of posts, a Reddit user had begun dumping huge amounts of code onto the subreddit r/A858DE45F56D9BC9. There was no explanation as to what the codes related to, and given the absence of promotion, it seemed like the posts would go unnoticed by most Redditors.
It's not even clear how the subreddit caught the attention of the site's wider userbase, but once the ciphers became more widely known, Redditors flocked to attempt to discern their true meaning.
A subreddit, r/Solving_A858, was formed in an attempt to get some of the best budding cryptologists together in the hunt for answers, united in their goal of cracking the elusive code.
At some point, the creator of r/A858 seemingly became aware of the growing attention surrounding his codes, and began dropping subtle hints including a message that, when decoded, revealed an ASCII drawing of Stonehenge. However, despite Redditors getting close to having fully cracked the truth of the online mystery, r/A858 went private in 2016 and has remained so in the years since.