The 8.9 earthquake in Japan was the largest in it's earthquake ridden history. The greatest and most far reaching danger may have been done to up to 5 reactors in 2 units at a nuclear power plant in the city of Fukushima, Japan when a backup power supply was damaged by a tsunami caused by the earthquake. Check out this post which gives an easy to understand and invaluable explanation of what and why it happened: Firedoglake.