ITT share strange/interesting internet subcultures or rabbit holes you've discovered. I'll start.
Geocities archives are fun to go through. Especially Geocities Japan, which shut down in 2019 as opposed to the rest of the world's Geocities which shut down in 2009. In fact, just about anything on archive.org is fun to look through, but I'm going to focus on the Geocities Japan archives.
https://www.archiveteam.org/index.php?title=GeoCities_Japan is a good place to start for both Japanese Geocities and Geocities in general.
It's a shame Geocities are now shut down globally, I would have loved to look through them if I were aware of their existence before they shut down in Japan.