|AT 18,700 FEET ABOVE sea level, the air is thin, odorless, and ether-clear. A gnarled fin of rock and ice marks the crater lip and summit proper of Citlaltépetl, better known as Pico de Orizaba, North America's third-highest peak and one of the tallest active volcanoes in the world. A step to the north, the mountain slides away 2,000 vertical feet in a sheet of glacial ice. A step to the south, a vertical headwall drops off into Pico's steaming crater a quarter-mile below.
For volcanologist Hugo Delgado Granados, the top of Mexico is where the research starts getting good. At 42, he has spent the better part of 20 years collecting data on the country's three highest volcanoes—Pico, 17,343-foot Iztaccíhuatl, and 17,887-foot Popocatépetl—a mission he describes as equal parts science and humanitarianism. Popo is located just 45 miles southeast of Mexico City and, in the event of a massive eruption, could blanket more than 400,000 people with deadly mudflows and pyroclastic avalanches of gas and debris. Since its last eruption in December 1994, Popo's crater has been boiling with magma, and the summit glacier has been shrinking by the day.
With any luck, Delgado's research will eventually insure that if "the big one" hits, no one will be in the way. When he can enlist a willing pilot, he helicopters onto the ash-strewn rims or flies planes through smoke plumes to measure temperatures and gather gas samples; on foot, he rappels 400 feet into craters and crevasses. In two decades of work, he's been rocked by seismic shock waves, temporarily blinded by ashfall, and choked by sulfurous smoke.
So when will Popo blow? Delgado shakes his head. "All the data tell us for sure is that Popo has much more to say."
Peter Lane Taylor's first book, Science at the Extreme, will be published this month by McGraw-Hill.