Lake Karachay
Lake Karachay is a radioactive lake in the former Soviet Union. A weapons factory called Mayak Facility suffered from a nuclear explosion here in 1957. The radioactive waves spread particles over 9k square miles.
The lake officially dried up in 1967, which sent radioactive dust particles another 900 square miles. The lake has some wet patches, which are highly radioactive. If you stand next to the lake for one hour without any protective gear, you will likely die.