When Mount Vesuvius erupted AD79, with a force 100,000 times that of the Hiroshima atom bomb, the citizens of Pompeii looked on aghast. They had no idea what had hit them. To the ancient Romans, Vesuvius was just a large mountain. They didn't realise that it was responsible for the earthquake that 17 years earlier had killed hundreds and partially destroyed their town.
The stunning column of fire and stone that came roaring out of the bowels of the volcano on that August afternoon AD79, "thrusting... bulging and uncoiling", as Pliny the Elder wrote, "as if the hot entrails of the earth [were] being drawn out and dragged towards the heavens", was totally unexpected. That's why the population died where they stood: gassed, pelted with pumice, buried in stone, sand and ash in the midst of their daily lives (as vividly described by Robert Harris in his new novel, Pompeii).
Now, 1,924 years later, we are much better informed about Vesuvius, but not necessarily a great deal wiser. This time around, nobody is going to get caught by an eruption the way ancient Pompeii was. Vesuvius, says Antonio Bassolino, the President of the Naples region, is "the most carefully monitored volcano in the world". Notice of the next great event should arrive some months before it happens.
But that's when the trouble will start. To understand why, it is enough to stand on the volcano's southern flank and cast an eye around. It's one of the most famous vistas in Italy, the stupendous purple volcano climbing up towards the sky, the sparkling blue of the Bay of Naples beyond, the towns and vineyards dotting the narrow plain in between. It has been the subject of a thousand prints and etchings, landscape paintings and picture postcards.
But today the whole region under Vesuvius - the Red Zone, as it is known - has become a huge overspill suburb for the city of Naples. Squashed between the volcano and the sea, modern Ercolaneo and Pompei, as well as other towns along the bay, have swollen into a single, densely congested urban mass. Altogether 18 towns hug the volcano's flank, and they have been growing with a zany, idiotic energy - and the encouragement of the authorities - since the last eruption in 1944.
Today the Red Zone is home to nearly 600,000 people. And it's this crazy overcrowding that has persuaded the Naples regional government to take drastic measures. It has set itself the challenge of persuading 150,000 of these people to move out of the shadow of Mount Vesuvius and make their homes elsewhere. This month it publishes a plan to offer €30,000 (£21,000) to any household willing to move.
It's not just the ranks of high, crude, concrete apartment blocks, lining the narrow streets all the way down to the bay, that pose the problem. Development continues far up the volcano's flanks. The slopes of the mountain constitute the Vesuvius National Park: no one's supposed to build here at all. But that hasn't stopped them.
Up here on the volcano's slopes, the architectural styles vary: bunker-like homesteads thrown together from breeze blocks, protected by dogs and surveillance cameras; blocks of flats that would not look out of place in town; a single elegant Modernist villa of plate glass and steel, set in a generous garden of oleanders and umbrella pines. The buildings steal up the steep slope towards the crater, like a mortal game of grandmother's footsteps.
All of them are illegal: abusivismo, as illegal construction is called, is the plague of modern Italy, ignored by weak governments, sponsored by organised crime gangs such as the Camorra (the Naples equivalent of the Mafia), and producing a sense of impunity in its perpetrators that grows bolder each time the central government declares an amnesty. A new one has just been announced. It's a lazy government's way of filling a budget hole in a hurry.
But when Vesuvius clears its throat again, all these people, legal or otherwise, will have to go. The tremors and the ominous belches from the volcano's depths will one day resume. The vulcanologists will study their data, draw their conclusions and make their announcement. And then the government of Antonio Bassolino will have to do something about it.
That means evacuating everyone in the Red Zone. Because, just as the destructive ferocity of a new eruption remains impossible to predict, there is also no way of knowing which towns will be in the firing line. As it did AD79, it will depend on the strength and direction of the wind. So everyone will be obliged to leave.
"In the event of an eruption," Antonio Bassolino told The Independent, "the authorities have the responsibility for arranging the evacuation of 581,000 residents and distributing them around Italy, at a cost to the public administration that could be as much as between €30m [£21m] and €40m [£28m] per day."
It's a challenge that would widen the eyes of a regional governor anywhere in the world, let alone in southern Italy, where respect for the forces of law and order is notoriously shaky. The last time the regional authorities attempted something of the sort, in 1980 - after scientists had predicted a violent eruption of a smaller volcano not far from Vesuvius, and the government ordered the evacuation of the town of Pozzuoli - there were violent clashes when residents refused to shift. The stand-off was only resolved after huge sums of money were found and a "special Pozzuoli law" enacted to build them a new town out of harm's way.
Three years later, a powerful earthquake in the same area led to rumours of an impending devastating eruption, and tens of thousands tried to flee in their cars, leading to long, petrified columns of vehicles unable to move in any direction. A number of people died from heart attacks because emergency services were unable to reach them through the snarl-ups.
That's the face of the new apocalypse confronting modern Pompei and the 17 other towns in the Red Zone: half a million panicked people in cars, fighting to get through the winding streets to safety. And that's why Antonio Bassolino has decided his government must act.
"Vesuvius is the most dangerous volcano in the world," he says, "on account of the number of people living under it. There is no active, dangerous volcano in the world that has such a population. So it is evident that the smaller the population, the better. The smaller the population, the smaller the danger Vesuvius represents; the smaller the population, the easier the plan of evacuation; the smaller the population, the easier to inform, to organise, to mobilise."
That's why Mr Bassolino has decided to pay people to go away. "In October," he said, "we will begin an information campaign to persuade 150,000 families to leave the area, with pamphlets, radio and television advertising and information to schools and colleges on why people should leave. We are hopeful that once some people decide to go, others will begin to imitate them." At the same time, he is hoping to persuade more tourists to come, and planning to convert the abandoned homes into guest houses and small hotels to accommodate them.
Isn't there a contradiction here: pleading with residents to leave because of the danger, then inviting tourists? "If we reduce the number of people living in the zone," Bassolino explained, "we reduce the time needed for evacuation. Because Vesuvius is today the most closely monitored volcano in the world, we will have a few months' notice of the next event. Tourists obviously will leave at the volcano's first cough. We have no difficulty in managing the movement of tourists. It will be a normal, natural, spontaneous event. Our problem is evacuating the people who live here."
But will they take the bait? I talked to a number of young people in and around modern Pompei - Mr Bassolino's prime objective is to persuade the young - and concluded that there is still work to do.
At a bar near the remains of old Pompeii, the barmaid dismissed the menace of the volcano. "Anything can happen anywhere," she said. "You can die in an accident in a car or a train. We could move to Rome and die in an earthquake. They could evacuate you - and you might end up dying sooner than if you'd stayed put. It's true that there are too many people living here: my big fear is if something happened and my little boy was somewhere else at the time. But we've got a house and a bar. We're staying."
"It's not going to happen," said Rossella Castellano, a travel agent, drinking a cappuccino in another bar. "Ninety-nine per cent of males in southern Italy are tied to their mother's skirts. They're not inclined to go anywhere."
"I look at Vesuvius as something that's been there for ever," said another woman at the same bar. "Two years ago we had a small earthquake here and a lot of people ran away, but they soon came back again. This town's got every sort of problem - no hospital, loads of unemployment. The way I see it, an eruption is the only thing that's going to solve our problems."
But eventually we found one person to back President Bassolino's campaign. Her name was Patrizia, and she lived with her husband and two children in an illegal building high up the volcano's slope. From her window you could distinctly make out the souvenir stalls on the crater's lip. The vast moonscape of the crater is quiet these days, emitting not even a wisp of smoke or a jet of stinking steam to impress the tourists. But from Patrizia's home it is very large, very close, and she knows that it is very far from dead.
Her apartment complex looks as if it was designed on the back of an envelope. The second floor is unfinished. Her neighbours are foreigners - Poles, Ukrainians, Croats. Her landlady and her family live in an almost equally ramshackle home a few steps down the mountain.
"The only reason we're here is because it's so difficult to find a place in Naples," she said. "I would leave immediately if I could. For the sake of my children and their future, I would love to get away - we'd be the first to go." Vesuvius gives her no peace. "Every time I go to bed I lie awake thinking, will we still be here in the morning?"