<P> Water from the Owens River filled the caldera to a depth of 300 metres (984 ft) as of 600,000 years ago . At that time, the lake surface was at an elevation near 7,500 feet (2,286 m). The lake drained sometime in the last 100,000 years after it overtopped the southern rim of the caldera, eroded the sill, and created the Owens River Gorge . A human - made dam in the gorge has created Lake Crowley, a partial restoration of the original lake . Since the great eruption, many hot springs developed in the area, and the resurgent dome has uplifted . </P> <P> During the last ice age, glaciers filled the canyons leading to Long Valley, but the valley floor was clear of ice . Excellent examples of terminal moraines can be seen at Long Valley . Laurel Creek, Convict Creek, and McGee Creek each have prominent moraines . </P> <P> In May 1980, a strong earthquake swarm that included four Richter magnitude 6 earthquakes struck the southern margin of the Long Valley Caldera . It was associated with a 10 inches (250 mm), dome - shaped uplift of the caldera floor . These events marked the onset of the latest period of caldera unrest that is ongoing . This ongoing unrest includes recurring earthquake swarms and continued dome - shaped uplift of the central section of the caldera accompanied by changes in thermal springs and gas emissions . After the quake, another road was created as an escape route . Its name at first was proposed as the "Mammoth Escape Route" but was changed to the Mammoth Scenic Loop after Mammoth area businesses and land owners complained . </P> <P> In 1982, the United States Geological Survey under the Volcano Hazards Program began an intensive effort to monitor and study geologic unrest in Long Valley Caldera . The goal is to provide residents and civil authorities with reliable information on the nature of the potential hazards posed by this unrest and timely warning of an impending volcanic eruption, should it develop . Most, perhaps all, volcanic eruptions are preceded and accompanied by geophysical and geochemical changes in the volcanic system . Common precursory indicators of volcanic activity include increased seismicity, ground deformation, and variations in the nature and rate of gas emissions . </P>

When did the largest swarm of earthquakes at long valley take place