Too many. I wish Lemmy had post tags, it would make things easier. There are communities I would be interested in, if I had any tools to help see the things I care about.
I'm left with more politics than I'd like, but that's still preferable to anything I block.
Broadly speaking:
Memes Anime, mostly due to all the anime girl communities Sports Foreign language communities if I really can't read anything, or I'm seeing too many posts in a row from them Communities that aren't for memes but that are overrun with them. Sometimes if this is driven by a particular user I might block them instead Communities with porn in the name
Not just air-tight, lighter-than-air-tight. I'm not sure if it's true for the whole set of lighter-than-air gasses, but helium is infamously difficult to keep from leaking, even with modern technology.
Good at making gasses behave is a weird choice for ancient technologies we lost.