Chernobyl Disaster, Soviet Union (1986)

A flawed reactor design and operator error during a safety test at Reactor No. 4 in Pripyat, Ukraine caused a steam explosion and graphite fire that released 400 times more radioactive fallout than the Hiroshima bomb, contaminating 150,000 square kilometers across Europe and causing an estimated 4,000 to 93,000 excess cancer deaths depending on the model used.