POMPEII,ITALY

The Vesuvius volcano did not form overnight, of course. In fact, scholars say that the mountain is hundreds of thousands of years old and had been erupting for generations. In about 1780 B.C., for example, an unusually violent eruption (known today as the “Avellino eruption”) shot millions of tons of superheated lava, ash and rocks about 22 miles into the sky. That prehistoric catastrophe destroyed almost every village, house and farm within 15 miles of the mountain.

But it was easy to overlook the mountain’s bad temper in such a pleasant, sunny spot. Even after a massive earthquake struck the Campania region in 63 A.D.–a quake that, scientists now understand, offered a warning rumble of the disaster to come–people still flocked to the shores of the Bay of Naples. Pompeii grew more crowded every year.

79 A.D.

Sixteen years after that telltale earthquake, in August 79 A.D., Mount Vesuvius erupted again. The blast sent a plume of ashes, pumice and other rocks, and scorching-hot volcanic gases so high into the sky that people could see it for hundreds of miles around. (The writer Pliny the Younger, who watched the eruption from across the bay, compared this “cloud of unusual size and appearance” to a pine tree that “rose to a great height on a sort of trunk and then split off into branches”; today, geologists refer to this type of volcano as a “Plinean eruption.”)

As it cooled, this tower of debris drifted to earth: first the fine-grained ash, then the lightweight chunks of pumice and other rocks. It was terrifying–“I believed I was perishing with the world,” Pliny wrote, “and the world with me”–but not yet lethal: Most Pompeiians had plenty of time to flee.

For those who stayed behind, however, conditions soon grew worse. As more and more ash fell, it clogged the air, making it difficult to breathe. Buildings collapsed. Then, a “pyroclastic surge”–a 100-miles-per-hour surge of super heated poison gas and pulverized rock–poured down the side of the mountain and swallowed everything and everyone in its path.

By the time the Vesuvius eruption sputtered to an end the next day, Pompeii was buried under millions of tons of volcanic ash. About 2,000 people were dead. Some people drifted back to town in search of lost relatives or belongings, but there was not much left to find. Pompeii, along with the smaller neighboring towns of Stabiae and Herculaneum, was abandoned for centuries.