The Fukushima nuclear power plant was damaged on March 11, 2011 by the Tohoku earthquake and accompanying tsunami. Wikipedia reminds us the incident was only the second time a level 7 nuclear event has occurred. (Chernobyl was the first.) The accident was not a single failure, but a perfect storm of failures and unlucky human responses.
The BBC reports that a leak of 300 tons of highly radioactive water escaped in this 4th documented leak at Fukushima. It is however, the largest recorded leak.
Because of the scale of the disaster, hundreds of containing pools were constructed to contain radioactive water that resulted when the plant operator had no choice but to cool the reactor core to avoid an unthinkable meltdown. It is these containing pools that continue to leak into groundwater and into the sea. The radioactivity is 8 million times greater than the safe limit for drinking water.
The real problem with nuclear power is there is no margin for error. Unfortunately us imperfect humans live in an imperfect world.