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 .
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 refuse to heed warnings, remembering a 1982 false alarm that damaged tourism and aggravated a recession. Afterward, journalists were accused of sensationalism and scientists of scaring people to get more funding. 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 probability of eruption is less than one percent.