In the late afternoon of 29 September 1957, a major explosion at the Soviet military nuclear facility "Mayak" located in Kyshtym in the Southern Urals, caused the contamination of an area of 20,000 square kilometers, home to 270.000 people. The Kyshtym incident was the first major nuclear accident in the world and is still considered the third most severe nuclear accident in history. However, it was kept almost completely secret by both the Soviet Union as well as by Western intelligence agencies such as the CIA until the end of the Cold War in 1989.