Most radioactive lake (ever)
Lake Karachay
4,440,000,000,000 megaBecquerels total number
Russian Federation (Ozersk (formerly Chelyabinsk-65))

The body of water most contaminated with radioactive pollution was Lake Karachay (aka "Reservoir 9"), a small lake formerly located near the Mayak Production Association, a nuclear facility in Chelyabinsk Oblast, Russia. Between 1951 and 1953, the open-cycle reactors of the Mayak Plutonium Plant disgorged high-level radioactive waste directly into the lake, and medium-level waste continued to be dumped there until the 1990s. A study carried out in 1993 found that the lake emitted 4,440,000,000,000 megaBecquerels (120 million curies) of radioactivity. The lake began to be filled in and concreted over in the mid-1990s, with the project warpping up in 2015.

The Mayak plant was established in 1945 as the primary production facility for the recently started Soviet nuclear weapons program. The crude reactors on the site were designed to create weapons-grade isotopes such as Plutonium-239 and Uranium-235. As they were part of a cold-war crash development program, safety was barely considered in their design. Initially the contaminated coolant water from the open-cycle reactors, as well as intensely radioactive waste created as a by-product of the extraction of plutonium, was dumped straight into the Techa River.

In September 1951 measurements taken in communities downriver of the plant revealed dangerously high levels of radiation, and so the worst of the waste was diverted to Lake Karachay, a small lake just to the south of the main facility. At the time the lake measured around 900 m in length and was about 500 m across at its widest point. Even after specialised storage facilities were created for this waste, other forms of waste continued to be dumped in the lake. It is not clear when this practice stopped – some sources say 1957, but a report published by a group of American scientists who toured the site in 1992 mentions waste still being dumped in the lake at the time of their visit.

The radioactivity of the lake is both in its water and in the layer of radioactive sludge that lines the lakebed. Measurements taken by that same group of scientists found that the earth around the lake had a specific activity of 740,000 megaBecquerel per kilogram, and that just by standing near the shore you'd be exposed to a dose of 5.6 Sievert per hour. That's enough radiation to kill you in about 50 minutes.