WHEN A VOLCANO GIVES CLEAR SIGNS, IT'S POSSIBLE TO avert a massive tragedy. On April 2, 1991, fissures opened on a remote flank of Mount Pinatubo, and steam exploded out. The area, 260 miles northwest of Mayon, was then inhabited by only a few indigenous Aeta people and guerrillas of the communist New People's Army, but the event was reported by Catholic nuns. Three days later, PHIVOLCS brought in a seismometer, and within 24 hours counted hundreds of volcanic quakes. Newhall's old friend Ray Punongbayan was so alarmed that he cleared out 4,000 people living within six miles. On April 23, Newhall arrived from the States with several VDAP specialists and 35 trunks of gear.

Newhall had researched what little was known about Pinatubo and classified it as a "tight" system. These tend to be contained for many years under a plug of cooled magma. Somewhere along the line, pressure rebuilds, and then one day it opens with incredible violence. During its last eruption, in around a.d. 1450, the mountain had totally blown itself up. Hence, it exhibited no pretty Mayon-like cone, just an irregular, hulking mass.

At the foot of Pinatubo lay Clark Air Base, then the United States' main Asian staging facility. Newhall called Clark's weatherman and explained that the 15,000 personnel and the fleets of warplanes could disappear if anything went wrong. Air Force brass were skeptical—they called the scruffy volcanologists "the beards" behind their backs—but gave them air support and a house to live in. When a helicopter ferried them to scout the mountain, a door gunner constantly scanned the ground with his weapon, watching for NPA. Upon landing, the rotor would wash back high elephant grass and the scientists would leap out, instruments at the ready for a quick in-and-out.

One of the first things Newhall spotted was a 230-foot-high cliff of crumbly brownish-gray material cut in cross-sections by a river bordering Clark. It was pyroclastic-flow debris. "Holy moly," he said. "When things go around here, they go big." By mid-May, the team had created a map showing 60 square miles of similar deposits.

Two weeks later, seismometers indicated that quakes were migrating toward Pinatubo's summit. Then a classic pre-eruption volcanic tremor started. During this period, Newhall took repeated helicopter flights to measure sulfur dioxide emissions with a correlation spectrometer lashed to the open gunport. Levels surged from 500 tons a day to 5,000—a sure sign of rising magma. At the start of June, the emissions suddenly dropped to near zero. Either the magma had backed off or the system was totally blocked—and building pressure.

Newhall and Punongbayan met with then–defense minister and future president Fidel Ramos, who listened gravely and then followed their advice for wider evacuations. On June 2, Newhall flew to brief generals at Pacific Command in Hawaii. He figured he would have time to continue on home for a quick visit and be back to catch any fireworks.

He was wrong. While he was in the States, the quakes accelerated. Authorities evacuated more people; eventually 200,000 would flee. On June 10, Clark was cleared. On June 12, a three-day series of pyroclastic blasts started. Orange lightning bolts generated by static electricity in the ash slashed the atmosphere. On June 15, seven great explosions evolved into one continuous, earthshaking roar. Pulverized magma shot 25 miles into the sky, raining darkness. Pyroclastic flows buried surrounding valleys 600 feet deep in hot debris. Then a typhoon swept in. The rain, combined with ash, crushed thousands of roofs far beyond the evacuation area. Lahars took out most bridges, and many villages, for 30 miles around.

When the air cleared, the top of Pinatubo was gone. After blowing out its guts, the summit had collapsed into itself, creating a half-mile-deep, 1.5-mile-wide caldera. Of the 20,000 people who had faced certain death, only about 100 Aeta and maybe a handful of guerrillas were killed in the eruption; no trace of them was ever found. (Two hundred other Filipinos died outside the evacuation zone in roof collapses.) No one was hurt at Clark, but hangars and other buildings collapsed. The U.S. moved out permanently; the remains of the base now comprise a modest business park. "This is the textbook case—a volcano that cooperated," says Newhall.

Three months after the eruption, a hot, acidic lake began collecting in the caldera. With the lake still steaming and occasional small explosions pocking the surface, Newhall had a helicopter fly over it, then had himself lowered down 300 feet on a cable to take important water and rock samples. He did not want the craft to go any lower, for fear that carbon dioxide, another common volcanic gas, might stall the engine. Since it also suffocates humans, he told the pilot to winch him up fast if he stopped waving or moving. He kept moving.