The 200-square-mile caldera is just south of Mono Lake, near the Nevada state line. The biggest eruption from Long Valley was 760,000 years ago, which unleashed 2,000 to 3,000 times as much lava and ash as Mount St. Helens, after which the caldera floor dropped about a mile, according to the U.S. Geological Survey.

Some of the ash reached as far east as Nebraska.

Long Valley's most recent eruption was in Mono Lake just 250 years ago, but it was very small. More worrisome is a swarm of strong earthquakes in 1980 and the 10-inch rise of about 100 square miles of caldera floor. Those developments have geologists concerned that Long Valley is gearing up for another eruption of some sort.

In the early 1990s yet another subtle sign of trouble became evident: Large amounts of carbon dioxide gas from magma below had begun seeping up through the ground and killing trees in the Mammoth Mountain part of the caldera. When these sorts of signs are present at a "central vent" volcano like Mount St. Helens, trouble is on the way soon. At a caldera, which has many outlets, it could mean trouble is years, decades or even centuries away, say volcanologists.

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