A lake once existed near the city of Oziorsk, south of the Urals, known as Karatchaï. It was used as a disposal site for radioactive waste from the Soviet nuclear facility called Mayak, starting in 1951. The lake, which was less than one square kilometer in size, became one of the most radioactive places on Earth. According to a report by the Worldwatch Institute, it contains 4.44 exabecquerels of radioactivity—far exceeding the 0.085 exabecquerel of cesium-137 released during the Chernobyl disaster in 1986. Scientists warn that spending just one hour near the lake’s shore could expose a person to a lethal dose of radiation. The Mayak complex, established in 1949, was central to the Soviet Union’s nuclear weapons program, producing plutonium for atomic bombs. Initially, the lake was intended as a temporary disposal site for liquid waste, but it remained in use for nearly four decades. Radioactivity levels in the lake worsened during a severe drought in the 1960s, which exposed radioactive sediments and allowed them to be carried by the wind, spreading contamination across surrounding areas. To reduce the spread of radiation, about 10,000 hollow concrete blocks were dumped into the lake between 1978 and 1986. However, this was only a partial fix, and the lake was finally buried in the 2010s. The final burial project involved filling the lake with concrete blocks, rocks, and a top layer of soil and rock, completed in December 2016. The site is now continuously monitored, with an annual budget of about 130.5 million rubles allocated to ensure its safety. Monitoring data show a significant drop in radionuclide deposition on the surface within ten months of the burial, though concerns remain about underground contamination. Karatchaï has been officially recognized by Guinness World Records as the most radioactive lake ever recorded. The contamination began during the operations of the Mayak complex, which suffered a major accident in 1957, greatly increasing the level of radioactivity in the area. For decades, the site was kept secret, and local residents were not informed of the dangers. Over time, the lake’s surface area shrank from about 0.5 square kilometers in 1951 to just 0.15 square kilometers by the 1990s. A severe drought in 1968 exposed radioactive sediments, causing widespread contamination across hundreds of square kilometers and affecting hundreds of thousands of people. The final burial involved injecting about 650 cubic meters of specialized concrete into the lake through 38 wells, with a total cost of around 17 billion rubles. The original lake no longer exists, now buried under layers of concrete, rock, and soil. However, radioactivity continues to affect the surrounding area, with contaminated groundwater extending about 4 kilometers from the former lake bed. Experts estimate that the pollution could take decades or even centuries to become inactive. The site is under strict surveillance by Rosatom, the Russian state atomic energy agency, and all economic activity is prohibited in the area.