On March 11, 2011, the magnitude 9.0 Tohoku Oki Earthquake rocked Japan and wiped out power sources for the Fukushima I Nuclear Power Plant. After a first, second, and then third failure of backup coolant systems caused by the earthquake’s subsequent tsunami that easily overwhelmed the plant’s sea walls, fuel rods melted in their reactors, the buildings that housed those reactors were crippled by massive explosions, and by March 15, 2011, unknown amounts of radioactive matter began irrepressibly flooding into the surrounding environment. For two months after the earthquake, the Japanese government and Tokyo Electric Power Company (TEPCO), the operators of Fukushima I, denied that any meltdowns had occurred at the plant.
Turns out, Fukushima Daiichi is the largest nuclear disaster since the Chernobyl incident of 1986. Possibly larger.