Sierra Nevada mountains at dawn, seen from Long Valley Caldera, Owens Valley, California. This view is from one of the many hot springs which demonstrate the volcanic activity giving the place its name. Long Valley Caldera is a depression in eastern California that is adjacent to Mammoth Mountain. The valley is one of the largest calderas on earth, measuring about 20 miles (32 km) long (east-west) and 11 miles (18 km) wide (north-south). Long Valley was formed 760,000 years ago when a huge vo