They died where they stood. Violently, with almost no warning. A highborn woman with her jewels. An armed soldier. A baby. Some were found in the rubble clinging to each other, as if for solace, in that awful moment when hot death came raging down the side of a mountain with the speed of a hurricane.

Almost 2,000 years ago a seaside town in southern Italy had the misfortune of being in the shadow of Mount Vesuvius—one of Europe’s active volcanoes—at the wrong time. Buried beneath 100 feet (30 meters) of sediment from that August night in A.D. 79, and from many more eruptions since, the 16,000 corpses of Herculaneum and neighboring Pompeii bear silent witness to the destructive force of volcanoes.

Objects of terror and fascination since the beginning of human time, they take their name from Vulcan, the Roman god of fire. Today some 550 are active across the globe, with 500 million people living in their dangerous shadows. At any given moment, somewhere between one dozen and two dozen are spewing ash and molten rock from the Earth’s belly.

Death by volcano can come in many forms. In 1815, history’s deadliest eruption claimed 92,000 lives in Indonesia. But only 12,000 died from the blast; the rest starved after their crops and livestock were destroyed. In 1883, 36,000 people died from a 100-foot- (30 meter-) high tsunami created by the eruption of Krakatau in the Sunda Strait between Java and Sumatra. A fast-moving cloud of hot gases killed 30,000 in Martinique during the 1902 eruption of Pelé, and in 1985, a tidal wave of mud from Nevado del Ruiz smothered and crushed 23,000 citizens of Colombia.