Russia's former Lake Karachay was an impressively polluted location, the early Soviet reactors were cooled with an open loop where highly contaminated water was discharged directly into the lake. The lake eventually became thoroughly sedimented with nuclear waste, and when levels dropped radioactive dust would be blown about the region. Apparently just half an hour on its shore would have been enough to doom you from the amount of radiation exposure.

They eventually filled the lake in, I can only say hats off to the poor buggers who had to do that. I think it's safe to say they had the world's worst job at the time.