IT's vile.
It does mean that finally I have a use for one of those swimming pool chlorine tablets I picked up off the road last year, so it smells bleachy.
I don't put weird stuff down there, so I'm assuming it's the stray tp or tampon... so I went at it with a plunger, but I don't know how to know whether or not it's cleared or not, since of course it doesn't drain out. Should I bucket out what's in there so that if it fills it won't overflow and give it a try? (When I get home from work, that is...) Thankfully, there's this working toilet down in the basement that two years of lack of use has not rendered busted.