It is Lake Karachay. And someone in Russia decided it would be a good place to dump their nuclear waste from a nearby storage facility.
The sediment of the lake bed is completely loaded with radioactive material. Far more than was ever released in Chernobyl, and in a smaller area too.
Due to a few “accidents”, they’ve taken steps to fill the bottom of the lake with concrete to reduce the fallout in the area. They claim to have significantly reduced it there but they will be studying it for the next decade.
But as per the Worldwatch Institute, Lake Karachay is still the most polluted open-air place on Earth in terms of nuclear waste.
If you stood on that shore without touching the water, you would get a dose of 600 roentgen in one hour, which likely means it would be your last day at the beach.
Don’t bother bringing sunscreen.