The first signs came not from the summit but from the seismographs hastily installed after those initial steam vents appeared in early April. Scientists recorded hundreds of small earthquakes every day. Radiocarbon dating of charcoal in old volcanic deposits revealed that Pinatubo had erupted catastrophically roughly 500, 3,500, and 5,500 years ago -- a cycle that suggested the current unrest was no minor event. By late May, sulfur dioxide emissions had climbed tenfold, from 500 tons per day to 5,000. Three concentric evacuation zones were drawn around the summit. Clark Air Base, the largest American military installation in the Pacific, sat inside the outermost ring. On June 7, a lava dome began pushing through the crater. Five days later, the first major eruption sent an ash column 19 kilometers into the atmosphere. Then came June 13, June 14, each day bringing larger blasts, each column reaching higher. The volcano was clearing its throat.

The climactic eruption began at 1:42 in the afternoon on June 15, 1991. What made it catastrophic was timing: Typhoon Yunya was passing just 75 kilometers to the north, wrapping the volcano in rain that mixed with volcanic ash to create a suffocating blanket of mud. The eruption column punched through the typhoon to a height of 34 kilometers. Pyroclastic flows -- superheated avalanches of gas and rock -- roared down every flank, traveling as far as 16 kilometers from the summit. The thermal energy released equaled 70 megatons of TNT. So much material was evacuated from beneath the mountain that the summit collapsed into a caldera 2.5 kilometers wide, reducing the peak from 1,745 meters to 1,485. Darkness fell across 125,000 square kilometers of central Luzon and lasted 36 hours. Ash fell on Vietnam, Cambodia, and Singapore. The volcanic cloud circled the globe, and over the following year, average worldwide temperatures dropped by approximately 0.5 degrees Celsius.

Eight hundred forty-seven people died -- a devastating number, but a fraction of what the toll could have been. Most were killed when rain-saturated ash collapsed roofs, a hazard amplified by the simultaneous typhoon. The evacuation that preceded the eruption saved tens of thousands. Volcanologists from the Philippine Institute of Volcanology and Seismology, working alongside scientists from the United States Geological Survey, had correctly read the escalating signals and pushed for evacuations that local officials initially resisted. The prediction was not perfect -- no one anticipated the coincidence of a major eruption and a typhoon -- but the monitoring, the evacuation zones, and the decision to act on incomplete data represented a landmark in eruption forecasting. At least 16 commercial aircraft suffered damaging encounters with the ash cloud after the June 15 blast, with ten engines requiring replacement, including all three on one DC-10. The Philippine Air Force's retired fleet of Vought F-8s, stored in the open at Basa Air Base, was destroyed.

The eruption did not end on June 15. In many ways, it had only begun. Valleys that once channeled rivers were buried under pyroclastic deposits hundreds of meters deep. Every rainy season for years afterward, lahars -- torrents of volcanic debris and water -- roared down the Tarlac, Abacan, Pasig-Potrero, and Bucao rivers, burying towns that had survived the eruption itself. In 1995, a lahar entombed the San Guillermo Parish Church in Bacolor, Pampanga, to half its 12-meter height. Eight hundred square kilometers of rice-growing farmland were destroyed. Nearly 800,000 head of livestock and poultry perished. The agricultural damage alone reached 1.5 billion pesos, and the region's gross domestic product, which had been growing at 5 percent annually, fell by more than 3 percent. Farmers adapted where they could, switching to quick-ripening crops -- peanuts, cassava, sweet potatoes -- that could be harvested before the late-summer lahar season. Scientists estimate the river systems will take decades to recover.

No community suffered more than the Aeta, the indigenous people who had lived on Pinatubo's slopes for centuries. Their villages were obliterated. When some returned after the danger had passed, they found their land buried under meters of ash and lahar deposits. Many were resettled in lowland communities, where they took up casual labor for farmers -- a profound rupture with the self-sufficient highland culture they had known. Aeta society became more fragmented, more dependent on and integrated with lowland life. The mountain they had known was gone, replaced by a scarred landscape with a caldera lake where the summit once stood. Today, roughly 500,000 people live within 40 kilometers of Pinatubo, including 150,000 in Angeles City and 30,000 at the Clark Freeport Zone, built on the former American air base. The eruption proved that the earth beneath central Luzon is not finished with its work -- and that the communities rebuilding in its shadow live with the knowledge that five centuries of quiet preceded the catastrophe of a single afternoon.