By Roland Pease
BBC science correspondent
Researchers are warning that the next eruption of Vesuvius could be much more deadly than the Italian authorities are planning for.
Footprints in ash deposits record people fleeing from the eruption
A US-Italian collaboration says deadly blasts have previously caused devastation which swept well beyond modern Naples.
Italian plans call for the evacuation of 600,000 people from the city.
But the researchers say up to three million people could be at risk according to the new assessment.
Vesuvius is best known for its eruption in AD 79, the eruption that buried the Roman town of Pompeii.
Plans for the evacuation of Naples are based on a similar-sized eruption in 1631.
But go back about 3,780 years and Vesuvius exploded with even greater force.
The new work, published by the US National Academy of Sciences, shows that in the Bronze Age, the most recent of those eruptions, hot, ash-ridden flows raced up to 25km (16 miles) to the north-west of the volcano, over and beyond modern Naples.
Anything within the first 12km (eight miles) would have been swept away by this pyroclastic surge.
To the east of the volcano, tens of centimetres of pumice rained down on the surrounding countryside - the weight would crush the roofs of modern houses.
The research has implications for the Naples evacuation plan
If the volcanic cycles of Vesuvius are anything to go by, a repetition of this eruption is as likely as a recurrence of the 1631 event - and Naples needs to realise how bad that could be, the authors say.
One of the authors, Michael Sheridan, told the BBC: "If we compare the eruption of the Bronze Age with the eruption that destroyed Pompeii, we can see that the deposits within Naples produced by the Bronze Age eruption range in thickness from three metres down to half a metre; whereas there are no really measurable deposits from the Pompeii eruption within the city of Naples."
Professor Sheridan said he had been motivated by the US experience of Hurricane Katrina last year, when authorities failed to prepare adequately for a well-understood weather disaster.
"The current planning doesn't consider the maximum probable event," explained Professor Sheridan.
"There have been notable cases recently where disaster planners have not taken into account the worst-case scenario and this eruption would certainly be one of those.
A archaeological dig 15km north-east of Vesuvius has uncovered victims
"It actually has a fairly high probability [of occurring] if we consider there have been eight large eruptions of this kind in the history of Vesuvius with a separation in time of 2-3,000 years, and it's been nearly 2,000 years since the last one."
Professor Sheridan recognised the difficulty of evacuating three million people from a major urban centre.
He noted that, as was the case after Hurricane Katrina, distribution of water, food, and housing for the survivors and the nature of the escape routes must also be carefully considered with an evacuation of such magnitude.