Within 20km (12 miles) of its crater live almost three million people - and every one of them, say the geologists, is at risk.

Their evidence comes from research, now completed, which shows that 4,000 years ago Vesuvius erupted with such ferocity that huge areas of land around the volcano were uninhabitable for decades.

Geologists say the evidence that has been uncovered since 2001 should have encouraged Italian authorities to change the evacuation plan they are rehearsing.

But vulcanologist Giuseppe Mastrolorenzo from the Vesuvius Observatory says civil emergency planners are still ignoring the worst-case scenario.

"It is politically negative to talk about the cataclysmic event," he says.

"The authorities have been told what could happen. But the evacuation plans they have in place still do not go anywhere near far enough."

Avalanche of ash

Mr Mastrolorenzo and his team took us to a disused quarry not far from Naples where they have been busy collecting samples.