Santorini, a volcanic island in the Aegean Sea, exploded violently around 1500 BC, wiping out the entire Minoan civilization. A volcanic-ash-rich mudflow swept down the slopes of Nevado del Ruiz in 1985, killing 23,000 people in Colombia. Oral and written histories are rife with tales of volcanic calamities that destroyed entire civilizations and killed thousands of people. But why do volcanoes occur and how do they work? Only recently have scientists...