California 's Long Valley is a 19-mile caldera created 730,000 years ago by an eruption 600 times larger than Mount St. Helens . In May 1989 , new underground lava movement began triggering thousands of tiny earthquakes and raising the valley floor . ``People who are frightened left long ago. Residents refused to heed warnings , remembering a 1982 false alarm that damaged businesses and aggravated a recession . Afterward , journalists were accused of sensationalism and scientists of scaring people to get more information . Currently , 5 - 10 small quakes happen daily as the Earth 's crust is stretched apart and magma fills half - mile - wide chambers four miles under the caldera , but the risk of eruption is less than one percent .
California 's Long Valley is a 19-mile caldera created 730,000 years ago by an eruption 600 times larger than Mount St. Helens . In May 1989 , new underground lava movement began triggering thousands of tiny earthquakes and raising the valley floor . Residents refused to heed warnings , remembering a 1982 false alarm that damaged businesses and aggravated a recession . Afterward , journalists were accused of sensationalism and scientists of scaring people to get more information . Currently , 5 - 10 small quakes happen daily as the Earth 's crust is stretched apart and magma fills half - mile - wide chambers four miles under the caldera , but the risk of eruption is less than one percent .