In the autumn of 1957, a mechanical failure at a secret nuclear facility in the South Urals led to a chemical explosion that contaminated 23,000 km² and affected more than 270,000 people. The incident, known as the Kyshtym accident, occurred on September 29, 1957, at the Mayak nuclear fuel reprocessing plant, located in the city of Ozersk. At the time, Ozersk was classified as a closed city and erased from Soviet maps to maintain secrecy around its military activities. The explosion resulted from a failure in the cooling system of a tank containing 70 to 80 tons of highly contaminated liquid radioactive waste, leading to a chemical explosion with a power comparable to 70 to 100 tons of TNT. The blast projected a plume loaded with radioactive isotopes, mainly strontium-90 and cesium-137, into the atmosphere. The affected area, later referred to as the radioactive trace of the Eastern Urals (EURT), spans the Tcheliabinsk, Sverdlovsk, and Kurgan regions. According to a report published by Greenpeace in 2017, up to 20,000 people are officially recognized as having suffered direct health consequences from this catastrophe. The Kyshtym accident is classified as level 6 on the international nuclear events scale, ranking it third among the most severe nuclear disasters, behind Chernobyl and Fukushima. The Soviet authorities concealed the accident for thirty-two years, censoring doctors and scientists, until its official recognition in 1989 under Gorbachev's glasnost policy. The incident was first revealed to the West in 1976 through an article published in the British magazine New Scientist by the dissident biochemist Yury Medvedev. His revelations, based on meticulous observations and cross-referencing, were initially met with skepticism by part of the Western scientific community. Entire villages deemed too contaminated were erased from official records, as if they had never existed. Doctors treating irradiated patients were formally forbidden from mentioning the word "radioactivity" in their diagnoses, and scientific publications that could mention the incident were systematically censored. The episode has left a lasting impression on nuclear safety experts, illustrating how state opacity can exacerbate the health consequences of a major industrial accident. The Kyshtym accident remains a powerful reminder of the dangers of secrecy in handling hazardous technologies and the long-term impact of nuclear disasters on communities and public health.