Sorey anticipates that the gas will continue to seep, probably for many more years. He knows only too well that the dormant volcano is tucked into a sprawling volcanic complex known as Long Valley - where, geologically speaking, there are no short answers. Signs on the shore of Horseshoe Lake warn visitors of an invisible danger at a once-popular campsite. The gas first bubbled to the surface in 1990, discovered when a forest ranger nearly passed out after taking shelter in a ski hut. As I discovered during my weekend there, Long Valley Caldera is aptly named.
Calderas are craters formed by the collapse of volcanoes.
This one is 20 miles long and ten miles wide. It was created 760,000 years ago when a gigantic eruption blew ash as far east as Nebraska and caused the earth to collapse a mile deep. The eruption, which sent magma gushing over the Sierra Nevada and down the San Joaquin River, is believed to have been one of the largest ever in North America. Bordered by the eastern slope of the Sierra, this volcanically restless area begins at Mono Lake and spreads 25 miles south past the town of Mammoth Lakes. Beginning as islands in the alkaline waters of Mono Lake, the Mono-Inyo chain ends at Mammoth Mountain, which has erupted frequently for 40,000 years and whose most recent eruption was 600 years ago.