DURING THE 20TH CENTURY, volcanoes killed 80,000 people, but not the way you'd probably imagine. Lava is actually a minor threat. It usually moves too slowly to catch anyone—only 100 or so people in the 1900s. Pyroclastic explosions like the one at Mayon are the top danger. Lahars—swift flows of volcanic mud—come in second. Falling ash, ballistic projectiles, poison gas, landslides, and volcanism-generated tsunamis round out the list.

Worldwide, 500 million people live in danger zones, but between eruptions most tend to forget. "Even a decade is a long time in people's memory," says Newhall. "It's just human nature." Earth has 1,500 potentially active volcanoes—ones that have erupted in the last 10,000 years—but during the past 200 years, three-quarters of the biggest eruptions have come at sites with no historically recorded activity. "A sleeping volcano is no cause for reassurance," says Newhall. "It may be building pressure. Bigger booms are typically preceded by longer waits."

Among the United States' 169 active volcanoes are dozens of dozing giants. The Volcanic Explosivity Index (VEI), a kind of Richter scale for eruptions that Newhall helped create, rates blasts on an exponential curve from zero to eight. When Mount St. Helens blew after sitting silent for more than a century, it knocked out a VEI 5—equal to one Hiroshima-size atomic bomb per second for nine hours. Other U.S. volcanoes that could erupt on this scale include the Three Sisters mountains, near Bend, Oregon, which alarmed geophysicists in 2001 when they realized the triplet peaks' flanks had inflated about four inches over four years (they have done little since); Washington's Mount Baker, which briefly melted off some of its glacial coating in 1975; and Oregon's Crater Lake and California's Mount Shasta, which haven't done much of anything lately—which is exactly the problem.

Then there's 14,410-foot Rainier, the dominant feature of the Seattle skyline and probably the greatest potential killer in the U.S. Though it's unlikely to erupt with St. Helens–like ferocity, Rainier's steep, loosely consolidated flanks are coated with some 36 square miles of glacial ice. Even a minor event—an earthquake, hot water bubbling to the surface, a squirt of lava—could produce lahars up to 100 feet tall and traveling downhill at 60 miles per hour. Some 150,000 people now live on lahar deposits, which in the past have reached Puget Sound, 40 miles from the summit.

Like other active U.S. volcanoes, Rainier is monitored with seismometers, which will warn USGS scientists of a possible eruption or rumblings powerful enough to trigger lahars. Last September, the USGS installed GPS units on the mountain to watch for bulges, which could indicate rising magma. If either system indicates trouble, scientists will alert emergency response teams, setting in motion evacuation procedures practiced once or twice a year in the towns lying in Rainier's lahar pathways.

The real nightmare scenario is a lahar unconnected to a quake or other detectable sign. This is of particular concern on the peak's western slope, where sulfuric acid has been eating away at rock layers from the inside. A collapse here could unleash a torrent of ground water and debris. In this case, the first indication of disaster will come from one of ten lahar monitors set in streambeds to pick up ground vibrations. The 5,500 residents of Orting—the closest town to Rainier, resting on deposits left by the last giant flow, 500 years ago—will have roughly 40 minutes to get to high ground.

For any volcano, researchers can calculate the average intervals between eruptions by studying deposits left by previous blasts. But Newhall doesn't labor under the misconception that these are so regular that a volcano is, at some definable moment, due for the Big One. To make a forecast, you have to listen.

"Luckily, magma has to make its way to the surface, and on the way, it makes noise," says Newhall. In a classic sequence, magma cracks through rocks six or 12 miles down, generating modest high-frequency earthquakes. When the magma gets closer to the surface, lower frequencies predominate—probably because of fluids gurgling around or gases coalescing into bubbles. The gathering pressure may deform or crack the surface; gases such as sulfur dioxide, a common eruption indicator, will emerge. Days or hours before an eruption, scientists may observe volcanic tremor—a continuous vibration, often with a steady, musical frequency.

To detect such signals, volcanologists are deploying ever more sophisticated instruments. These include growing seismic networks, satellite imaging systems that note surface bulges as small as an inch, and light sensors that can detect sulfur dioxide from miles away. Unfortunately, data from all these devices rarely provide clear answers. A snoozing volcano can snort, twitch, and turn over for months or years, then go back to sleep. Or explode.

"A volcano can tease you. It'll make you think it's going to blow up, and then at the last moment it'll back off," Newhall says. "This leaves us with an interesting dilemma. For a myriad of reasons, most people resist leaving home without an immediate cause. But if you wait until you're absolutely sure, you'll probably be too late. If you warn too soon, people stop listening to you. Generally you only get one chance."