MOUNT ST. HELENS NATIONAL VOLCANIC MONUMENT, Wash. - In the 20 years since Mount St. Helens' last eruption, scientists have turned the volcano into a living laboratory - monitoring every little twitch under the surface.
But with all their high-tech gear, they cannot be sure they will get as much notice before the next cataclysmic event.
In fact, scientists speculate that a huge chunk of Mount St. Helens - or worse, Mount Rainier, which looms over populous valleys outside Seattle - could collapse without warning as magma eats away at their inner cores like termites on an old house.
That could happen next week or next year, or perhaps centuries from now.
So as survivors and rescuers gather this anniversary week to share memories from the eruption, scientists continue to cautiously watch the Cascade Range's most active volcano and use their limited knowledge to help protect people from other volcanoes around the world.
Ed Klimasauskas, a geologist at the U.S. Geological Survey's Cascades Volcano Observatory, compares the ongoing research to wearing a seat belt.
''You might never need it, and probably never will need it. But it's good to have,'' he says.
That was never more clear than on May 18, 1980.
At 8:32 a.m. that Sunday morning, two months after earthquakes signaled the awakening of the sleeping giant, Mount St. Helens' summit and much of its north flank crumbled in a huge landslide that was the largest in Earth's recorded history. The landslide depressurized the volcano's magma system, triggering massive explosions that ripped through the sliding debris.
Inside the 230 square-mile ''blast zone,'' enough trees to build 300,000 homes were blown down like toothpicks, and lakes and rivers were buried, flooded or moved. A column of ash shot 15 miles into the air, completely darkening Spokane, Wash., 250 miles to the east.
When it was over, 57 people had died, along with countless animals.
The mountain's beautiful snowcapped cone, which peaked at 9,677 feet before the eruption, was gone. In its place was an ugly, gaping crater. The top 1,300 feet of the mountain lay on the valley floor, as barren as a moonscape.
These days, scientists know if Mount St. Helens so much as shudders in the wind.
Sensors placed on and around the now-8,363-foot volcano track minor earthquakes - constant reminders of the mountain's active core. Tourists at the Johnston Ridge Observatory, named after a government scientist killed seconds after the eruption, get excited when needles on the visitor center's seismographs start twitching.
''They want to know whether that means the mountain is going to erupt today,'' Klimasauskas says with a grin.
Other sensors monitor water levels in lakes at the mountain's base and the snow level behind the lava dome that now bulges out of the crater. A slight shift of the mountain could send the growing snowpack down in a huge landslide, flooding valleys downstream.
A computerized system detects lahars, or volcanic mudflows of snow and loose rock debris. And geologists use lasers to keep an eye on growth of the lava dome.
''Electronically, we're taking the pulse of a restless volcano,'' Klimasauskas says.
Scientists also are working on a geologic map. They are studying lava flows over the past several thousand years in hopes of detecting an eruption pattern. Ideally, they would like to be able to predict Mount St. Helens can be expected to erupt in, say, 120 years, but that is not yet possible.
''I think it's fair to say we have come a long way in terms of our monitoring technology to determine what happens when volcanoes are restless, and how to forecast eruptions,'' says C. Dan Miller, a fellow USGS geologist.
''But we just don't have the ability yet to accurately forecast what a volcano is going to do.''
Local experts hone their skills by responding to eruptions around the world.
The USGS's Cascade observatory is home base for the federal government's Volcano Disaster Assistance Program, which can deploy a team of scientists within 48 hours. The group currently is responding to volcano emergencies in Ecuador and Mexico, says Miller, the team's chief.
What scientists have learned from Mount St. Helens and other eruptions is allowing them to take steps to raise awareness about the delicate nature of Mount Rainier, a 14,411-foot volcano near the Seattle metropolitan area.
Geological evidence shows that Mount Rainier has generated huge avalanches and mudflows that reached 50 miles or so to Puget Sound and Seattle's outskirts, according to the USGS. And the mudflows don't appear to have resulted from eruptions, so scientists wonder if Mount Rainier will give as much notice - if any - as Mount St. Helens did in 1980.
''Mount Rainier is one of the most high-risk volcanoes we have at this point, and it's one we devote a lot of attention to,'' says Miller, who was on his way to an observation post when Mount St. Helens erupted. The post was obliterated.
The 1980 Mount St. Helens eruption demonstrates ''the reality that there are a bunch of young volcanoes in the West that will erupt again and again in the future,'' he says.
Scientists are now researching worst-case scenarios involving ''non-eruptive events,'' such as the collapse of a flank. Mount St. Helens helps because the inner walls and structure of its crater are accessible.
Scientists also are learning to interpret a volcano's actions.
For example, the first sign of activity before the 1980 Mount St. Helens eruption was a series of earthquakes two months earlier. By May 17, more than 10,000 earthquakes had shaken the volcano and a bulge had grown out of the north flank.
But volcanoes do not always advertise their intentions so far in advance.
Klimasauskas cites Alaska's Mount Redoubt, which was shaken by earthquakes in December 1989, and was in a ''full-blown'' eruption 36 hours later.
Mount Adams, a rural 12,276-foot peak in Washington that apparently has not erupted for at least 1,000 years, grabbed the USGS' attention in fall 1997 with huge landslides that came without warning.
''It re-emphasized the point that sometimes chunks of volcanoes just fall off,'' Klimasauskas says.