archive.org/download/usenet-alt/alt.hypnosis.mbox.zip (90mb)
archive.org/download/usenet-alt/alt.gay.hypnosis.mbox.zip (500kb)
There's others (alt.mind.bending, alt.mindcontrol) but as far as I can tell they're pure garbage. You can see the full alt.* list <<archive.org/download/usenet-alt|here>> and download whatever else suits your fancy.

After extracting I can view the larger mbox files (with great difficulty) in Notepad++. I hear vim works great on large files too but I don't have that available. alt.hypnosis.mbox is 339,305,626 lines total. There must be something good in there.

Meanwhile, I'll be reading alt.sex.fetish.robots

So what exactly are in these?

Discussions on hypnosis, mostly non-erotic as far as I can tell except for the gay one. Also spam. Posts date from 1992 to 2011, in reverse chronological order, so the most interesting stuff (before the spamming caught on) is at the bottom.