Mt. Pinatubo: Scientists’ Countdown to Eruption : Volcano: American and Filipino experts, satellite photos and a computer named Porky produced accurate warnings.

Several hundred yards from a tree-shaded officers’ housing complex at the northern edge of Clark Air Base, once America’s largest overseas air base, Bella Tubianosa hikes down to the end of the world.

Smoldering mud and ash clog the huge river valley up to 20 feet deep. Searing steam hisses up near flash-incinerated logs. Everything is deathly gray, every plant withered and dead. The ground is almost too hot to touch.

Climbing atop the infernal torrent, the 27-year-old geologist scrapes some mud aside with a small shovel. She jabs a special thermometer six inches in and reads the gauge.

“It’s 185 degrees Celsius,” she says. That’s 368 degrees Fahrenheit, much higher than boiling temperature. “It’s much hotter deeper down.”

At midafternoon, half the sky is dark. Rooster tails of light gray ash waft down, like gentle snow, peppering visitors with soot. A giant mushroom cloud rises in the distance. Eight and a half miles away, Mt. Pinatubo is erupting again.

“It’s only a minor ash emission,” she says calmly.

If the scene appeared surreal, studies here in the weeks before Pinatubo’s disastrous June 15 eruption--one of the largest and most violent in history--helped produce a scientific success story.

It was here that a dozen or so Filipino and American geologists, seismologists, geophysicists and others began to map the long-dormant volcano’s last blasts. Together with satellite photos, seismographs and a computer named Porky, this helped them track and predict the volcano’s fiery reign with almost uncanny precision.

“The success rate is certainly not 100% in these things,” said Jim Mori, 36, a U.S. Geological Survey seismologist from Caltech. “But this one, I think, we’ll put in the win category.”

Thanks to the scientists’ warnings, tens of thousands of Americans and Filipinos were evacuated to safety before Pinatubo began blowing its top. It ultimately spewed an estimated 2 billion tons of volcanic debris, raining ash across central Luzon. About 300 people died, but the death toll could well have been far worse.

The tale of Pinatubo began April 2, when a Catholic nun from a mountainside convent reported explosions and steam coming off the summit. Word quickly reached the Philippine Institute of Volcanology and Seismology, about 50 miles south in Quezon City.

A seismic station was hurriedly set up about five miles from the 4,975-foot summit. Until it malfunctioned, the suitcase-sized machine measured and radioed back 493 volcanic quakes in the first 25 hours. It meant that rock was fracturing deep inside the mountain. Helicopter surveys showed that five vents had opened on the northwest side.

As the volcano continued to rumble and vent steam, a three-member Geological Survey “volcano crisis team” arrived April 24 and helped install seven portable seismometers on the volcano’s flanks.

None of the team had even heard of Pinatubo two weeks before.

The Americans set up offices with the Philippine scientists in two three-bedroom condominiums at Clark Air Base. They unloaded trunks of equipment, including computers code-named Porky, Willie and Squab.

“At that point, we were looking at lots of earthquakes,” said John Power, 31, a geophysicist from the Alaska Volcano Observatory in Fairbanks. “The real tip-off was the magmatic gases with lots of sulfur dioxide (in the steam). They told us we had magma moving at shallow depth.”

Magma is molten rock from deep in the earth. When the buoyant, gas-filled material rises, it creates titanic pressure. Sitting atop Pinatubo was a “plug dome” of hardened magma from the last eruption six centuries earlier.

The volcanic activity picked up sharply in late May. Then, in early June, the seismograph needles began clattering with up to 2,000 volcanic tremors and quakes a day. An explosion rained ash nearby.

Most worrisome of all, tiltmeters on the mountain and aerial photos indicated bulging of the slopes and a dome of viscous magma building just below the summit. It quickly grew to more than 300 feet long and 90 feet high.

“That’s when we knew we had a Pelean volcano,” said Delfin Garcia, 35, a Philippine seismologist. “It’s the most destructive kind of volcano.”

Pelean volcanoes are named for Mt. Pelee, which exploded in Martinique in 1902 and killed 36,000 people. Instead of spitting fiery lava, which flows at less than a mile a day, Pelean volcanoes explode in deadly avalanches of superheated gas, steam and rocks--called pyroclastic flows, literally “fiery rocks”--that roar down the slopes at speeds up to 100 m.p.h.

The geologists had reached the same conclusion. Carbon-dating of long-dead vegetation, now charcoal, embedded in strata and outcrops along river valleys indicated the volcano’s prehistoric fury. Together with studies of shifting magnetic fields in the rock, the data indicated that pyroclastic flows occurred about 600, 2,000 and 4,300 years ago.

“The reconstruction was pretty hurried, but a picture was emerging of periodic, very violent eruptions,” said Ed Wolfe, 55, a geologist from the Cascades Volcano Observatory in Vancouver, Wash.

The geologists quickly created a “hazard map,” with bright yellow marking pyroclastic danger zones. Nearly the entire western slope was yellow. So was a tongue that extended far to the east, down the Sacobia River, to the edge of Clark Air Base. A separate yellow tongue aimed down the Abacan River to Angeles, a city of 300,000 outside Clark.

On June 7, Philippine officials formally warned that the volcano could erupt within 24 hours. More than 11,000 people from villages to the north and west were ordered evacuated. Cars, trucks and buffalo carts streamed off the mountain.

The U.S. team, meanwhile briefed Clark’s commanders, Maj. Gen. William Studer and Col. Jeffrey Grimes, and pointed out the danger from a helicopter. They also arranged to meet Angeles’ mayor, Antonio Abad Santos, but he sent his secretary instead and ignored the warning.

At 2:55 p.m. June 9, there was a series of small eruptions. Ash-laden steam clouds billowed skyward.

It was enough. At dawn the next day, the order was given: Evacuate Clark of 15,000 Air Force personnel and dependents. With the mountain pouring smoke behind them, the convoy of evacuees drove 48 miles to Subic Bay Naval Base. The Angeles mayor accused the Americans of “overreacting.”

But two days later, early Wednesday, the first major eruption started. As earthquakes rattled the region, smoke and steam soared in a mushroom cloud 65,000 feet high. Pumice and ash began pelting terrified villagers.

“In one sense, the team was rather relieved,” said Mori, the Caltech seismologist. “They had called it right. It made believers out of a lot of nonbelievers.”

Eruptions increased all day Thursday and most of Friday. Radar measured ash clouds 80,000 feet high. Mud, boulders and pyroclastic flows roared north and west down four river valleys. Three seismographs were destroyed. Philippine officials ordered further evacuations as ash poured down.

Then late Friday, inexplicably, the volcano grew quiet.

“We waited and waited for hours,” said Richard Hoblitt, 47, a volcanic stratigrapher from Vancouver. “It didn’t make sense.”

The answer came about 3 a.m. Saturday. The “cyclops,” an infrared imaging device at Clark’s aircraft control tower, picked up not one large plume of smoke, but scattered and increasingly powerful plumes and explosions. Seismograph needles banged from side to side, registering seemingly endless earthquakes.

“The plug dome was starting to fall apart,” said Hoblitt. “Magma was pushing up. The cork was coming loose.”

At 5:55 a.m., Pinatubo’s top began to literally explode. One by one, three of the team’s remaining four seismographs went silent. The scientists were figuratively blind.

“All we could tell was something big and nasty was happening,” said Hoblitt. “It’s raining rocks. A typhoon is coming. It’s pitch dark. There’s lightning everywhere. We knew this was it.”

The group retreated first to an office complex on Clark’s eastern side. At 2 p.m., with an even larger eruption under way, the scientists and 1,500 remaining Air Force personnel at Clark fled to an agricultural school far from the mountain, abandoning the giant base.

Rocks and coarse ash rained down for hundreds of miles, burying Clark and Subic in heavy, chalky muck. Mudflows destroyed three Abacan River bridges in Angeles. A pyroclastic flow filled the Sacobia River behind Clark. Hundreds of thousands fled in panic, and an estimated 250,000 lost their homes or businesses.

Today, Pinatubo continues to puff and rumble. A mile-wide smoking crater is all that’s left of the summit.

Although another major eruption appears unlikely for now, river valleys and ravines are filled hundreds of feet deep with pyroclastic material that blankets Pinatubo almost to the bottom. Officials warn that monsoon rains may trigger huge landslides and mudflows.