Exposure may occur from nuclear fallout when nuclear weapons are detonated or nuclear containment systems are compromised. During World War II, the United States Army Air Forces dropped atomic bombs on the Japanese cities of Hiroshima and Nagasaki, leading to extensive contamination of food, land, and water. In the Soviet Union, the Mayak industrial complex (otherwise known as Chelyabinsk-40 or Chelyabinsk-65) exploded in 1957. The Kyshtym disaster was kept secret for several decades. It is the third most serious nuclear accident ever recorded. At least 22 villages were exposed to radiation and resulted in at least 10,000 displaced persons. In 1992, the former Soviet Union officially acknowledged the accident. Other Soviet republics of Ukraine and Belarus suffered also when a reactor at the Chernobyl nuclear power plant had a meltdown in 1986. To this day, several small towns and the city of Chernobyl remain abandoned and uninhabitable due to fallout.