Japan's tsunami disaster took an even scarier turn Saturday when an explosion rocked one of the country's nuclear reactors, blowing the roof off the building containing it and knocking down walls, and causing an unknown amount of radiation to leak. Pressure at the Fukushima nuclear plant began rising when the 8.9 earthquake cut off power to the reactor's cooling system. Pressure is now at twice the normal levels, despite venting slightly radioactive vapor. Residents living within 12.5 miles from the reactor have been evacuated. People within 16.2 miles from the reactor have been told to remain indoors. Officials say a meltdown is possible, but that it wouldn't affect residents outside the evacuated radius.