Take a scenic flight over the summit of Mount Vesuvius in Italy, and the view below is chilling. A dense patchwork of urban sprawl from the nearby city of Naples laps at the flanks of one of the most violent volcanoes on Earth. Since A.D. 79, when Vesuvius exploded with little warning and entombed Pompeii and its 3,000 townsfolk under 15 feet of scalding ash, the volcano has erupted at least 30 times. In Pompeii the destruction was so complete that nothing was known of the once-bustling Roman city until archaeologists rediscovered it 1,600 years later.

Blame Earth´s explosive nature on what lies beneath its
surface. Oceans of seething magma below the planet´s crust can top 2,000˚F. The intense heat forces rising magma into the crust; if enough magma collects near a weak point, it bursts through to the surface. For centuries, this is exactly what´s been happening on Vesuvius, which today has more than two million people
living in its shadow. â€We know Vesuvius is capable of a major eruption,†says William Menke, a professor of earth and environmental sciences at Columbia University. What we don´t know is when. Hoping to fathom a guess, scientists at the Vesuvius Observatory have long monitored seismic sensors on the volcano that record tiny rumblings inside the mountain. A case of seismic hiccups might mean that an eruption is looming.

On the other hand, a volcano may be on the brink of eruption without emitting even a single detectable tremor. Perhaps today´s most useful tool helping scientists determine when a mountain is about to awaken is satellite-based InSAR (Interferometric Synthetic Aperture Radar). Since the early 1990s, InSAR-equipped satellites have been firing staggered beams of radar waves toward Earth. When the waves bounce back, InSAR records their signal intensity (a measure of the wave´s reflection versus absorption) and its phase (how long it takes the waves to make a round-trip). With InSAR, scientists can track tiny deformations in the Earth´s surface that occur in the absence of seismic activity.

InSAR is particularly useful for studying volcanoes in countries that tend to be ignored by the usual parade of visiting Western
scientists. Although peaks such as Vesuvius, Mt. Rainier in Washington State and Japan´s Mt. Fuji garner a lot of attention, there are dozens of other volcanoes just as deadly in Central and South America and Indonesia. The Gede volcano is only 40 miles from downtown Jakarta and its population of nine million. The hill country around Gede has become a favorite weekend getaway for locals-there´s even an 18-hole golf course on the volcano. Popocatepetl, at 17,930 feet, soars over Mexico City, a mere 37 miles away. The â€Smoking Mountain†is known to have erupted 36 times and sputters steam and gas from its summit almost monthly. Atitln Caldera is about 75 miles from Guatemala City. Take a stroll in the city, says Stanley Williams, a professor of geology and volcanology at Arizona State University, â€and you realize that all the rocks you are standing on came from Atitln. If that eruption happened today, it would probably kill 90 percent of the population of the country.†For any of these volcanoes, predicting an eruption using InSAR depends on groundcover-rocky or barren surfaces tend to produce better images than snow- or tree-covered terrain. Residents of Naples will be glad to know that Vesuvius is perfectly photogenic.

And, thankfully, recent InSAR images show very little swelling. Not that Naples is in the clear, warns Claudio Prati, an electrical engineer and professor at the Polytechnic University of Milan, where he specializes in InSAR research. â€When it comes,†he says, â€the eruption will be very, very fast and explosive.â€