"They dropped everything and started running," Sheridan said.
At one site near the modern town of Nola, archaeologists have found dinner dishes abandoned on a table. They also found the skeleton of an abandoned dog, plus nine pregnant goats still in their pen.
The skeletons of two people buried in ash indicate that even leaving possessions behind didn't give everyone time to escape.
Disaster planners had long presumed that the modern city of Naples (see map) was far enough away from the volcano that only its southern suburbs were at risk.
But geologists now report that they have found thick sediments from the Bronze Age blast beneath the heart of the modern city.
The threat had not previously been recognized, because it is difficult to do geological studies in cities, where rock outcrops showing sediment layers are beneath buildings and pavement.
One of the best ways to study a city's geology is to look at outcrops revealed during construction projects. When Italian scientists started doing this in Naples, they found bands of blast deposits up to ten feet (three meters) thick.
The 1980 blast that leveled forests on Mount St. Helens in Washington State produced similar deposits. But those were only three feet (one meter) thick.
Any blast that could produce three-meter deposits would be strong enough to devastate even the city's biggest structures, Sheridan said.
Although Vesuvius has been active at many times in its history, geologists have found traces of eight titanic eruptions.
"These eruptions are separated by 2,000 or 3,000 years," Sheridan says.
Then comes the kicker: "It's been 2,000 years since the last one. And the current hazard planning is not taking these major events into account."
But Vesuvius isn't the only hazard Naples faces. The island of Ischia, in the Gulf of Naples, is a volcano. So is Campi Flegrei, a highland on the city's west side.
Campi Flegrei is a particularly dangerous type of volcano, capable of producing a massive explosion, said Giuseppe De Natale.
De Natale, a research director at Italy's National Institute of Geophysics and Volcanology, spoke about the volcano during the May 2005 meeting of the American Geophysical Union.
Such eruptions are rare, De Natale said, but "second only to a large meteor impact" in terms of damage.
Cities on Volcanoes
Worldwide, several major cities lie in the danger zones of volcanoes. Managua, Nicaragua, is actually built inside a crater. And the Seven Hills of Rome are all volcanic.
The dangers are enough that hundreds of volcanologists and disaster planners met this January for a "Cities on Volcanoes" conference in another volcanic city: Quito, Ecuador.
Grant Heiken, a volcanologist based in Washington State, has written several books on volcanoes and is a member of one of the commissions that helped organize the conference.
"There are 60 cities with populations over 100,000 with volcanic hazards that we know about," Heiken said.
"Two are megacities: Mexico City [Mexico] and Manila [in the Philippines]. Something on the order of 80 million to 100 million people are at risk."
These risks take on new urgency in the aftermath of Hurricane Katrina, which devastated communities and left thousands dead or displaced along the U.S. Gulf coast last August.
Evacuating 200,000 or 300,000 people is one thing, Sheridan said. But Naples has three million residents. Even those not at direct risk from a blast would still lose their lifelines to important services.
"You have to plan," Sheridan said.
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