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.