Not strictly an "internet" mystery, but I fondly remember the "excursions" old /x/ did into Worlds.com - at least before some LARPers had to fuck it up with cringy 2spooky4u shit.
For those who don't know it, Worlds.com basically was one of those early 2000s 3d online chatrooms, composed of several of these eponymous worlds. It was founded in 1994, and still is maintained as of today so its parent company can file lawsuits against any and all MMORPG publishers because it thinks it holds a patent on "3d virtual worlds".
Anyway, the "game" itself has a skeleton staff of former users/admins who are still chatting away on it, and these in itself are harmless. More interesting where the worlds itself - aside from conventional chatrooms, there was stuff like promotional worlds for The Blair Witch Project, one for DMC and Aerosmith, and so forth.
Even more interesting were user-made worlds, which generally required you to know their respective "coordinates", or a friend who did and who subsequently invited you to them. As whatever construction kit Worlds offered seemed to be reasonably powerful, many of these were pretty amazing - I remember stumbling upon a giant mausoleum some built for a user who passed away during the game's heyday, but also a ballbusting gallery (as in, a literal gallery full of framed ballbusting pics), a "hell" world full of SM toys, and then some. Sadly, the few remaining admins quickly set these worlds to be off-limits - presumably to hide whatever embarrassing stuff former users may have left behind.
Still, hells know whatever you might have found in there if you had known the necessary coordinates.