The Kyshtym disaster, sometimes referred to as the Mayak disaster or Ozyorsk disaster in newer sources, was a radioactive contamination accident that occurred on 29 September 1957 at Mayak, a plutonium reprocessing production plant for nuclear weapons located in the closed city of Chelyabinsk-40 in Chelyabinsk Oblast, Russia in the Soviet Union. An improperly stored underground tank of high-level liquid nuclear waste exploded, contaminating thousands of square kilometers of land, now known as the Eastern Ural Radioactive Trace (EURT). The matter was covered up, and few either inside or outside the Soviet Union were aware of the full scope of the disaster for nearly two decades.