Imagine living in a place where, in the past 25 years alone, communities have been wiped out by lava flows; eruptions have flattened huge expanses of land and killed people miles away; and avalanches of volcanic debris have swept people to their deaths.

A small portion of the snow-covered flanks of Mount St. Helens is visible at right as the sun rises over the fog-filled valleys visible from the Castle Lake Viewpoint, Friday, May 6, 2005, in Washington state. Officials opened up the Johnston Ridge Observatory and several other areas near the volcano to the public Friday that had been closed since volcanic activity increased last fall. (AP Photo/Ted S. Warren) (Ted S. Warren - AP)

If you live in America, you live in such a country.

The United States is among the most volcano-rich nations on Earth -- home to 45 eruptions and 15 cases of notable unrest at 33 volcanoes since 1980. But while a handful of hazardous mountains are relatively well-laden with monitoring equipment, many dozing giants are beyond scientists' electronic eyes and ears, posing a significant threat to thousands of people, according to the first comprehensive assessment of U.S. volcano risks.

And although about half of the 169 U.S. volcanoes capable of erupting are in relatively remote regions of Alaska, the danger that even they pose to Americans and others is not as small as many might suspect, says the report by scientists at the U.S. Geological Survey, the research arm of the Interior Department. That is because the plumes of ash that can spew undetected for many hours from unmonitored volcanoes are largely invisible to commercial airliner pilots and their radar but can quickly cause jet engines to fail.

Nearly 100 commercial jets are known to have inadvertently flown into volcanic ash plumes since 1980 at altitudes as high as 37,000 feet, including eight in which one or more engines shut down. Three 747s lost all four engines.

In every case, pilots managed to restart their engines -- albeit after harrowing drops in altitude. But those warnings should not be ignored, said geologist John Ewert, who prepared the report with co-workers Marianne Guffanti and Thomas Murray. "Three 747s have been turned into gliders, okay? And they are really lousy gliders."

The new report ranks every U.S. volcano on 15 "hazard" measures -- including history of activity and explosive potential -- and 10 "human exposure" measures, such as proximity to population centers and flight routes.

The team then calculated the level of monitoring each volcano deserves and performed a "gap analysis" that compares those levels to current monitoring levels.

Monitoring typically includes specialized seismographs that, unlike earthquake monitors, are tuned to detect vibrations caused by underground magma flows; ground- and satellite-based systems to detect bulges in the earth just a few inches high that typically precede eruptions; and devices to detect gas emissions and changes in groundwater levels or chemistry.

The team found that monitoring is strong, though has room for improvement, at the three U.S. volcanoes now erupting (St. Helens in Washington, Kilauea in Hawaii and Anatahan in the U.S. Pacific territory of Northern Mariana Islands) and the two volcanoes that seem closest to erupting: Mauna Loa in Hawaii and Spurr in Alaska.

It found worrisome gaps for 13 other "very high-threat" volcanoes and 19 high- or moderate-threat volcanoes that, despite posing serious dangers to aviation, have no real-time, ground-based monitors.

An additional 21 volcanoes -- including some in California, Washington and Oregon and the Yellowstone caldera in Wyoming -- pose lesser threats but also deserve better monitoring, the report says, in part because of their proximity to population centers and because many are capped in snowfields that, upon melting, would cause deadly debris flows called lahars.

The new analysis (at http://pubs.usgs.gov/of/2005/1164/ ) could not have been written 25 years ago, when an exploding St. Helens triggered Earth's largest landslide in recorded history. But scientists have since learned much about eruption prediction -- in part by studying the 23 smaller eruptions at St. Helens through 1986 and the current one, which started in October.

One lesson is that a volcano can come very close to erupting without seismic warnings, yet seismic activity can also go on for years without it meaning an eruption is imminent. Thus it pays to deploy many kinds of instruments.

"There's no such thing as an 'eruptometer,' " said Guffanti of USGS's volcano hazards program. "You want these different things that all together can say, 'Oh my god, we're having an eruption!' "

Automated monitors are also crucial because many dangerous volcanoes are out of view and inaccessible for much of the winter.

"We want to be proactive and not have to drop down on a parachute and play catch up," Guffanti said.

For about $15 million, the most hazardous volcanoes could be wired up into a National Volcano Early Warning System, consisting of sensors that feed into the nation's five volcano observatories, according to the USGS.

The system could also help prevent unnecessary and costly evacuations when frightening activity can be found to pose no imminent risk, as turned out to be the case with the 1996 rumblings beneath Akutan in the Aleutian Islands.

"That magma came up so close to the surface of the earth it actually formed ground cracks," Guffanti said. But the agency correctly assured residents that they were safe.

The aviation industry is a prime supporter of a warning system. Tens of thousands of people flying between North America and Asia pass over volcanoes in the Aleutians and Marianas every day. Yet when Anatahan erupted in the Marianas in 2003, five hours went by before airlines were notified.

"It only takes five minutes for that ash to get up to 20,000 feet," said Terry McVenes, a pilot and executive air safety chairman for the Air Line Pilots Association.

Ash particles are hard as metal and have caused hundreds of millions of dollars in damage to aircraft. They abrade windshields so badly, they leave pilots blinded. Their static charges can wipe out navigation systems. Worse, they melt inside hot engines, sticking to fan blades and clogging cooling vents, causing engine shutdowns.

"You can easily drop 10 to 15,000 feet getting everything restarted," McVenes said.

"There's an investment that was made in the science after Mount St. Helens erupted in 1980," Ewert said -- an investment that allowed scientists to issue warnings in advance of last year's eruption and alert air traffic controllers within five minutes of the first ash release.

"Volcanoes always give you some warning," Ewert said. But somebody, he said, has to be listening.