Japan told to prepare for massive eruption.
Prof Hiromu Okada, of Hokkaido University, said evidence suggests a major eruption could take place before completion of the lava dome, which appears to be nudging through Mount Usu's surface. Experts worry that such a powerful eruption could by accompanied by a pyroclastic flow, a searing mix of gas and rock that races down the mountain incinerating everything in its path.
Lives are not in danger, since all residential areas in the danger zone have been deserted. But hot springs resorts and farms - the backbone of local economy - could be destroyed.
Already, video taken from helicopters show roads and buildings that have been ripped apart as magma moving underground causes the surface to crack.
Fearing a prolonged evacuation, officials are having temporary housing shipped in from around Japan. "We don't know for sure what's going to happen, but we have to prepare for the worst," said a government official.
Mount Usu is one of several volcanoes on Japan's northern island of Hokkaido. It coughed up gas and debris last week for the first time in 22 years.
Rain fell on Wednesday night, and officials worried that accumulated ash and rock would come sliding down the mountain. But the downpour was lighter than expected and there was no evidence of damage to towns caused by massive mudslides.