I ARRIVED IN THE PHILIPPINES ON A MONDAY MORNING TO stay with Newhall and his wife, Glenda, at their newly built house in the hamlet of Salvación. It's near the southern end of Luzon, the archipelago's main island, along a spectacular bay. From the back porch, looking nine miles across the water, we could see Mayon venting a constant bluish cloud. It's been restless lately, spouting eruptions small and large since 2000.

The house sits on ancient volcanic deposits, but Newhall assured me Mayon's modern eruptive pattern does not reach here. "At least as far as I can tell," he said. "There is always a plus or minus with any measurement."

Newhall grew up in Quincy, a logging town in the Northern California Sierra Nevada. Early on, he collected unusual rocks and got some faint sniffs of hydrogen sulfide from volcanic Lassen Peak, where his family visited often. He earned a geology degree from UC Davis in 1970, then joined the Peace Corps. He wanted to do something positive for the world and did not want be drafted into the Vietnam War, which he detested. He ended up teaching Geology 101 at a college in Legazpi, a city near Mayon. He loved it. On weekends he traveled around the volcano's slopes, mapping deposits and clambering into the then-cool, inactive crater. He met Glenda, a local teacher, early on and they married in 1971.

Hoping to combine science with humanitarian aid, in 1980 Newhall completed his Ph.D. at Dartmouth and joined the USGS. For much of his career he was based in Washington State, where he and Glenda raised two kids. He frequently jetted to volcanic trouble spots—Ecuador, Papua New Guinea, Mexico, Indonesia—becoming a major force at VDAP. In the 1980s, he compiled a first-of-its-kind list of the threats posed by the world's so-called supervolcanoes—the mountains geologists believe could blow big enough to affect global climate and kill huge numbers of people. Lately, he's been developing an Internet database that will catalog information from eruptions around the world, telling scientists how particular sequences of indicators played out in the past.

Since Newhall and Glenda resettled in Salvación, much of his pension has gone to help put ten sons and daughters of poor neighbors and Filipino friends through college. Glenda, now a nurse, dispenses free medicine and advice. They also have a family of 11 living in an annex to their house; the parents needed employment, so the Newhalls hired them and put them up.

On my first night in Salvación, I heard Newhall's cell phone chime in the adjoining bedroom at 1:31 a.m. In the morning, he showed me a text message from Ed Laguerta, a PHIVOLCS volcanologist: BULUSAN EXPLOSION ON FLANK. AM HERE AT BULUSAN NOW. Bulusan is a volcano a few hours' drive south from Mayon that explodes moderately about once a decade. A big blast there could reach towns containing some 56,000 people.

After a morning of bouncing along Luzon's potholed roads, we came across a half-dozen PHIVOLCS staffers working from a tiny concrete-block observatory in a goat pasture. Several miles off lay Bulusan, a jungly sawtooth-shaped mass jetting steam from a crack near its bald top. Bella Tubianosa, the resident volcanologist and an old friend of Newhall's, told us seismometers had picked up quakes a few days before. Then explosions shook the area, and a dusting of ash floated down onto nearby towns.

"This is a crisis, but so far it's a minor crisis," said Tubianosa, a slender woman in her forties who projected the calm of a surgeon.

Tubianosa had two cell phones, which kept ringing. A TV news crew came in. In the glare of their lights, she squelched a rumor that PHIVOLCS had called for an evacuation. Afterwards, she took me aside. "Right now I'm feeling a lot of concern, because this volcano is not showing its usual activity," she said. Behind her, the observatory's old-fashioned seismograph drums turned slowly, their narrow ink traces showing that the quakes had abated. As a rule, decreasing seismicity is good. On the other hand, noted Newhall, "When it gets quiet, that can mean magma is right at the surface—there's nothing in its way."

At least four seismometers are needed to precisely locate volcanic quakes, but Tubianosa used only two, which was all PHIVOLCS had here. The Philippines owned one correlation spectrometer, a device that detects sulfur dioxide, but it was at Mayon. For an alternative, Newhall e-mailed a Maryland-based NASA scientist with access to a satellite that can spot the gas from space. He got back an image showing a cluster of red pixels downwind of Bulusan. This, he said, represented about 1,000 tons of sulfur dioxide—proof that magma was indeed somewhere inside the mountain. Tubianosa looked at the image intently. "It doesn't tell us how far down," she said.

We drove to the village of Sangkayon, nestled along the volcano's base, where a fine whitish ash had coated rooftops, trees, and yards and was fueling a dust cloud on the well-traveled road. Many people had pulled shirts or surgical masks over their faces. Newhall interviewed two young men who told him the volcano had roared for ten minutes and a reddish column had lit the sky. We drove upward along a dirt track and found two other men with much the same story. They added that they had smelled something like matches being lit—evidence of magma near the surface.

We parked along a rice paddy with a view of the steaming peak. "So," said Newhall, "the question, as always, is this: Is the volcano clearing its throat and getting ready to say something important? Or has it already said its thing?"