At 01:23 AM on 26 April 1986, reactor 4 of the Chernobyl Nuclear Power Plant in Soviet Ukraine exploded during a safety test. The explosion and subsequent fire released large quantities of radioactive particles into the atmosphere, spreading across much of Western USSR and Europe.
The disaster forced the evacuation of 350,000 people, including the entire city of Pripyat. It is considered the worst nuclear disaster in history and was a significant factor in the dissolution of the Soviet Union.