LEE VINING, Calif. - This eastern Sierra valley looks utterly serene now a spacious concavity lying between pine-clad peaks, silent save for the chittering of an occasional ground squirrel or the cry of a golden eagle.
But about 760,000 years ago, Long Valley had a very bad day. It exploded in one of the most spectacular eruptions of recent geologic time, throwing 780,000 cubic yards of magma and rock into the atmosphere.
The plume from the explosion went so high that deposits of ash have been found in Nebraska and traces have been discovered in the eastern Pacific. Gigantic molten flows deposited massive windrows of lava and ash scores miles long to the south.
After the explosion, the Earth subsided into the magma chamber, creating a depression measuring 10 by 20 miles - the current Long Valley. Nothing so cataclysmic has happened here since - which isn't to say things will always remain tranquil. In fact, the contrary seems assured.
Magma chambers are still down there, like abscesses in an old tooth - and magma is charging them. Sooner or later, geologists say, some or all of the magma gathering beneath the valley will find a route to the surface.
Although it's unlikely to match the fireworks of the one 760,000 years ago, the chances of an eruption in the Long Valley area are roughly the same as the chances of a large earthquake occurring on the San Adreas Fault.
But that doesn't seem to faze residents of this region, who seem blithely resigned to - or unaware of - the dangers beneath their feet.
The main evidence for continuing volcanic activity in Long Valley is the rapid rising of a "resurgent dome." The floor of the valley is ballooning upward as it is pushed from below by the increasing quantities of magma.
Through the late 1970s to 1980, scientists with the U.S. Geological Survey were alarmed to note the dome had risen by more than 8 inches.
A series of magnitude 6 earthquakes in May 1980 convinced them an eruption could be imminent, and they issued a warning to local residents. But the eruption didn't happen, leading some locals to accuse the service of Chicken Little hysteria.
That's unfortunate, given that the dome continued to rise steadily from 1980 through 1997. It has since been more or less quiescent, but four years isn't much in geologic time; the disturbing fact remains that the resurgent dome is about 32 inches higher than it was in the 1970s.
Another sign of volcanism can be found at Horseshoe Lake, a popular recreation site near the ski town of Mammoth Lakes, where large numbers of trees have died from subterranean carbon dioxide releases. A cross-country skier was killed by the gas while touring the lakeshore in 1998.
Long Valley is a caldera, which means it is vastly different from the single-vent volcanoes familiar to most people.
Calderas are far less common than single-vent volcanoes - and when they explode, the effects are far more spectacular. Short of an impact by a large asteroid or comet, caldera explosions are arguably the most disastrous natural event that can befall the planet.
"These caldera-forming eruptions are huge and unprecedented in recorded time," observed David Hill, the USGS scientist in charge of the agency's Long Valley observatory.
Caldera events occur when the central part of the land mass sitting on a large magma chamber abruptly subsides downward as a single block. The resulting pressure ejects the magma with convulsive, explosive force.
Surveillance of Long Valley remains intense, but the data don't seem to mean much to local residents. Indeed, Mammoth Lakes, one of the West's premier ski destinations, is booming.
A reprise of the caldera's collapse would do more than depress local property values. A significant portion of the east side of the Sierra would be obliterated, and communities hundreds of miles away in both California and Nevada could be inundated in snowstorms of ash.
Global climate would be significantly affected, lowering temperatures throughout the world - much as the last explosion caused an extended cooling period on Earth. Precipitation patterns might become altered, and agricultural production likely would suffer.
What can be done to minimize the impacts of caldera eruptions? Beyond evacuating people in the path of imminent destruction, not much.
"These events are so big that we would be hard pressed to respond meaningfully to one, even if we knew it was imminent," Hill said. "What could we do? Tell farmers in Iowa they'd have to pack up for a few years?"
Still, the USGS maintains a response plan for the Long Valley area. Earthquakes around the valley are generally seen as evidence of magma movement. A temblor swarm with at least one magnitude 5 quake triggers a "yellow" warning from the agency. Increasing seismic activity means that the magma is moving close to the surface and warrants an orange warning. Red means an eruption is under way: Get out and stay out.
Volcanic activity will continue to shape the Long Valley region. New eruptions could come anytime. "We have measured precise changes in the gravity field (below Long Valley) that tell us magma is being added to this system," Hill said.
But the next eruption is likely to be less spectacular than the one that shook the region long ago. More probably it will resemble the ones that created a series of relatively small cinder cones and craters in the vicinity of Mono Lake during the last 5,000 years.
"We've been able to do (subterranean) seismic imaging that tells us there probably isn't a single huge magma chamber capable of producing an eruption equivalent to the one 760,000 years ago," Hill said. "Rather, it seems there are a number of smaller chambers one to two kilometers in diameter that likely will produce smaller eruptions."
But it's important to keep the risk in perspective, Hill added. "We live in a part of the world that is geologically active. Volcanism is just part of life along the Pacific Rim."