BULUSAN KEPT THROWING OFF GASES AND TREMORS. AFTER a day of watching, we returned to Mayon. While driving the road circling the base, we came upon three boys digging fill from a road cut that exposed about 15 gravelly layers of lapilli, or fallen volcanic stones. Each layer represented a major eruption.

"There's the 1814 one," said Newhall, pointing to an eight-inch swath near the top from a blast that killed 1,200 people. He dug out a thumb-size stone. "If that fell on you from a mile up, it would put a hole in your head. All these places around here are going to be hit again someday."

Outside the city of Legazpi, we visited the ruins of Cagsawa, a village buried by the pyroclastics and lahars of 1814. It's now a tourist site, with souvenir and refreshment stands. About the only thing visible of the old village is the steeple of a Spanish colonial church; the rest of it lies below, along with the people who were praying inside at the time.

Newhall pointed to the six-mile expanse stretching from Cagsawa to Mayon's summit. Fields, roads, and houses have sprung up on top of the eruption deposits. Otherwise-landless subsistence farmers are attracted by the volcanic soils, which need no fertilizer because mineral nutrients get replaced every time the volcano vomits. All told, some 400,000 people are in reach of Mayon. "The root problem is not volcanism—it's overpopulation," he said.

Indeed, no matter how good forecasting technology gets, human factors will continue to cause calamities that could have been avoided. In 2002, the Congo's Nyiragongo volcano blew during a chaotic civil war, leveling much of the city of Goma, uprooting 400,000 people, and mowing down 150 with a rare, fast-moving type of lava flow. Several years before the eruption, Goma's small volcanic observatory had been rocketed and pillaged. When Newhall arrived with colleagues in the aftermath, he learned that Congolese scientists had tried to warn the populace, but without a working government or telephones, they didn't get far.

Others refuse to take heed because it's inconvenient. In 1982, after quakes shook the popular California mountain-resort area of Long Valley, the USGS issued a low-level eruption warning. There was no evacuation, but vacationers were badly spooked; condo sales plummeted, and storefronts went vacant. Furious locals nicknamed the USGS the "U.S. Guessing Society." C. Dan Miller, the volcanologist in charge, considered moving his family after a local suggested that a bomb could wind up in his car.

Around Mayon, people simply accept the danger. In 2000 and 2001, PHIVOLCS successfully called evacuations just before blowouts, and only livestock were killed. However, a typhoon hit on November 30, 2006, dumping 18-plus inches of rain onto the mountain within a few hours. Lahars killed more than 1,000 people, but the ruins were quickly reoccupied.

One hot afternoon in Bonga, I was sitting with Alan Nuñez and his family under the shade of a broad-leafed malabago tree next to his tiny house. "This is where our livelihood is, our memories," said Hercolano Nuñez, Alan's father. "How could we leave?"

Hercolano told a story about a man his grandparents knew, who saw an uson coming during the eruption of 1897. The man tried to escape on his horse, but he was so scared, he forgot to unhitch the animal from the tree where it was tied. When he whipped it, man and horse spun in concentric circles—"One! Two! Three!" Hercolano shouted, running his hands in circles—while the uson burned them alive. Everyone under the malabago guffawed.