Located next to a Soviet's plutonium fuel processing center, Lake Karachay a small lake in Central Russia was considered the world' most contaminated place from the 1950s to 1960s.
Coinciding with the nuclear arms race during the cold war, the nearby nuclear facility was over capacitated with reprocessing demand and ran out of room for waste storage. Their solution? Officials decided to dump all liquid waste into Lake Karacha and hoped that one day they would relocate the waste to a permanent storage location. But things got out hand quickly (of course they did). Before they realized, the lake had become too contaminated for anyone to get close to.
It was estimated that at its peak the lake accumulated about 4.44 exabecquerels of radioactivity, mostly from beta particle emitting isotopes such as cesium-137 and strontium-90. In comparison, what was in the lake even outweighed the total amount of long-lived radioisotopes released by the Chernobyl disaster. The hourly dose rate by the lake was so strong that it could potentially kill or severely harm anyone working there.
Source: SciShow via Youtube