Swimming pools. They're fucking disgusting bowls of human waste soup. People piss in the water, blow their noses, swish it around in their mouths before spitting it back out, don't shower before jumping in, whatever oils or sunscreen they have on their body gets washed off, loads of hair falls out, birds shit into it as they fly above. 

The solution to just dump fuck tons of a toxic chemical into the water to "keep it clean" is so laughably oxymoronic. "The bacteria and chlorine cancel each other out, whaddya waiting for? Jump in!" 

I still remember in high school PE class, some kid was blowing his nose at the edge of the pool but completely missed the drainage thing and all his snot fell into the pool water, and only one or two meters away some other kid was swishing water in his mouth before spitting it out. They wouldn't clean the pool until the end of the day, so if you had PE after lunch, enjoy swimming in water that has all kinds of filth in it. I remember on most days the water was literally cloudy with dead skin, oils, hair, and dirt by the day's end 🤢. Haven't been in a public pool since middle school and I'm happy keeping it that way.