On March 11, 2011 at 2:46 PM a magnitude 9 earthquake hit Japan. The resulting tsunami knocked out power to the cooling systems of the Daiichi nuclear reactors. Without cooling, the reactors began to overheat and go towards meltdown. In an attempt to cool them down, the government ordered seawater to be pumped into the reactors, but they were to late.
Infographic on the events of the Daiichi nuclear disaster
A infographic showing how nuclear meltdown works