When you think of nuclear disasters, you think of the Chernobyl plant in Ukraine and the Fukushima plant in Japan, and maybe Three Mile Island. But after Fukushima and Chernobyl, the third biggest nuclear disaster is called Kyshtym. Never heard of it? That’s because it happened in 1957, at the height of the Cold War, deep in the eastern Ural Mountains of the Soviet Union. The Soviets disclosed details to no one, not even the people affected. Even the name is a misdirection, because it didn’t happen in Kyshtym. It was in the town of Chelyabinsk-65 (which was renamed Ozyorsk in the early 1990s); this town, according to the Soviets, did not exist.

The Mayak Production Association runs plutonium facility No 817 in the Chelyabinsk region of Russia. During the Soviet era, its location was a secret to anyone who didn't work there. It was known as Chelyabinsk-40 after the postal code, and the nearby community was named Chelyabinsk-65. The facility was built in a hurry just after World War II in order to catch up with the Americans in nuclear weapons technology. The plant, which included six reactors, processed nuclear materials to develop weapons-grade plutonium. At the time, relatively little was known about the effects of radioactive materials on human workers, and even the dangers that were known were disregarded by Soviet authorities in their haste to develop nuclear weapons. The future town of Ozyorsk grew up around the facility.

The plant was dangerous from the beginning. Radioactive waste was disposed of by dumping it into the Techa River. Solid waste was dumped on-site, and smoke was released into the air with no regard to its contents. Protective gear for the workers was minimal, and most of it was believed to have been done with forced labor from local prisoners. The first recorded nuclear accident there occurred in 1953, but went unnoticed until a worker developed radiation sickness (his legs were eventually amputated due to radiation burns, but he survived). Four other workers were affected as well. It was the first of dozens of incidents at the facility that continued for decades.

On September 29, 1957, one of the Mayak plant’s cooling systems failed. No one noticed until it was too late. A waste tank exploded, sending a cloud of radioactive material into the air, which fell over an area of 20,000 square kilometers. Though 270,000 people lived there, only 11,000 were evacuated (and that took up to two years to accomplish). Those who remained were pressed into service to clean up the debris by destroying contaminated crops and livestock. They worked without protection from radiation, and then they went back to their homes.

The Soviet reaction was a puzzle to many of the peasants who lived near the Mayak plant. In the village of Korabolka, farmers thought a global nuclear war had begun when they saw the explosion. Within a few days, 300 of the village’s 5000 residents died of radiation poisoning. An evacuation was planned, but only ethnic Russians were relocated. The remaining half of the village were ethnic Tatars, who were left in place. In the more than 50 years since, many villagers are convinced they were left as an experiment. The cancer rate for Korabolka, now called Tatarskaya Korabolka, is five times that of an uncontaminated village. Other villages around the region report elevated rates of cancer, genetic abnormalities, and other illness.

The Western press knew very little about the event. There were rumors, but no concrete details until 1976, when biologist and Soviet dissident Dr. Zhores Medvedev published an account of the disaster in New Scientist. As late as 1982, scientists in the West displayed doubt that contamination in the area was due to a nuclear accident instead of industrial pollution. Information came out in small pieces until the fall of the Soviet Union.

The Kyshtym disaster is not the only reason that Chelyabinsk is so contaminated. The waste that was dumped in the Techa River from 1949 to 1956 still claims victims in the villages downriver. In Muslumovo, those who weren’t evacuated in the 1950s and '60s were left in place and were tended to by national radiation experts who were studying the subjects of “a natural experiment” to glean information on the effects of nuclear war on humans. The villagers were not told of the research, and were kept in the dark as to why so many of them were sick. It was only in 1992, when Soviet records were declassified, that the nature of the Muslumovo experiment was uncovered. Even then, one pediatrician estimated that 90% of the village’s children suffered from genetic abnormalities, and only 7% were considered healthy.

The Mayak plant stopped processing weapons-grade plutonium in 1987, but still operates in Chelyabinsk, reprocessing spent nuclear fuel shipped in from all over Russia. The safety features of the plant have been vastly upgraded from those of Soviet era. The current level of danger from radiation at the plant is in dispute.

The area around the reactor site has been called the most contaminated place on earth. The villagers who were affected by the disaster and the long-term industrial pollution are still fighting for relocation and compensation. We will probably never know how many people died from nuclear contamination, for several reasons, and it's also difficult to pinpoint the extent of the contamination a half-century ago. And Soviet suppression of information and documentation makes current research on the incident extremely difficult. Even today, Russia doesn't welcome challenges to its official version of the story.