Do you ever just click one video and then let the YouTube autoplay take you down a weird dangerous route? Dangerous to your sleeping hours, that is. I think that was probably where I first heard about Cicada 3301 (and Thinking Sideways covered it, like Elisa Lam. I swear, I listen to other podcasts). But this one, while still unsolved, is a little more upbeat.
Well, if you consider dark-web puzzle solving upbeat.