It is just about impossible to imagine the terror of those living among the perfumed lemon groves and in the picturesque seaside villages south east of Naples when scorching pyroclastic flows swept down the steep slopes of Vesuvius in 1631. Some 5,000 people were burned alive. Yet people returned soon after the disaster, rebuilding and cultivating the land anew.
The cycle of volcanic eruption, devastation, and re-colonisation of the area around Vesuvius has been played out repeatedly since the Roman towns of Pompeii, Herculaneum, Oplonti and Stabia were buried in 79AD. Most recently, in 1944 when Vesuvius last erupted, lava flowed into the village of San Sebastiano forcing inhabitants to abandon their houses and flee.
Within a decade, though, that same stretch of coast between Naples and Sorrento and areas to the east were being developed as never before.
There are now about 600,000 people living at Vesuvius' foot, says director of the Vesuvius Observatory in Naples, Lucia Civetta. And the volcano is far from extinct. There is still fumeralogical and seismic activity. It is not a question of whether Vesuvius will erupt again, merely when.
Yet, Civetta notes, 'amazingly, 10 to 15 years ago, only a few people realised that living at the foot of Vesuvius was high risk'.
Vesuvius is the quintessential volcano. Eruption tends to be plinian, with magma and gases pushed up from a chamber some 10km beneath the earth's surface through a single large fissure.
'Vesuvius erupts explosively, ' notes Civetta. 'A dense mixture of super-heated gas, liquid and magma rises up in the atmosphere at high speed and mixes with surrounding air, producing an eruptive column up to 50km high, ' she explains.
Ash and dust from the column is blown eastward by prevailing winds, typically carried over 40km. In the past, ash has fallen in densities as great as 40kg/m 2, crushing buildings and smothering everything else. Heavy rains immediately following volcanic activity, when the huge amounts of vapour condense, cause mud flows, particularly severe to the north of Vesuvius.
'If the density of the eruptive column is greater than that of the atmosphere, though, it cannot rise but collapses, ' Civetta continues. The resulting pyroclastic cloud then flows down the slopes of the volcano at speeds that can exceed 100km/h. With a temperature of 5000C or more, it incinerates everything in its path.
Civetta's concern is not with Vesuvius alone. The western Pozzuoli district of Naples, home to 500,000 people, is built on the Campi Flegrei caldera - a huge volcanic depression.
Calderas are formed when the roof of a shallow magma chamber collapses following eruption. Campi Flegrei's magma chamber is far larger than that of Vesuvius and, at 4km below the surface, far shallower. Over time it has refilled, resulting in two periods of ground resurgence, from 1969-72 and 1982-84. In Pozzuoli harbour where impact was greatest, a combined 3.5m of uplift was measured.
'There is a very large question over how many more increases in ground level can take place before fractures occur, allowing magma to rise to the surface, ' Civetta frowns. 'Campi Flegrei last erupted in 1538, which makes it the more likely of the two volcanoes to erupt next. It has been at rest for as long as it ever has in the past. Any future eruption is likely to be explosive.'
Since it first became active 37,000 years ago, Campi Flegrei has erupted at between 50 and 60 different locations. It is impossible to predict where magma will next burst through the earth's crust. But when it does erupt the Observatory has calculated Campi Flegrei will impact on an area roughly double that in danger from Vesuvius.
A large shoulder of tuff separating Pozzuoli from the rest of Naples will save the majority of the city's population from pyroclastic flows. But prevailing winds will swamp the whole of Naples in thousands of tonnes of solid magma, dust and ash.
There is as yet no emergency plan in place to deal with an eruption of Campi Flegrei. But anxiety over the rapid pace of urbanisation combined with memories of Vesuvius' last eruption has led to development of an evacuation plan for the area east of Naples. Work started in 1993 and a final strategy is to be presented to Italy's government early next year.
Working with the Department for Civil Protection, Vesuvius Observatory has calculated that with the combined use of increased frequency train services on the rail line connecting Sorrento with Naples, special buses, and requisitioning passenger ferries for sea evacuation, clearing people from the area around Vesuvius will take seven days.
'We would be able to get people out of the danger zone faster with road improvements.
There's essentially just one road out which gets extremely congested every day, anyway, ' Civetta adds. Doubling the highway from Naples to Salerno has been proposed but would be difficult to achieve in an area 'which is densely populated and where the morphology is so steep'.
Efforts are now focused mainly on raising awareness of the evacuation procedure. The heads of each of the 18 communes in the affected area have been briefed to spread information and there are annual trial runs - the last, in October, involved 1,000 people. Scaling up operations to cope with 60,000 will require a different order of organisation, Civetta admits. 'The purpose of the trials is to attract the attention of the whole community to the problem of evacuation.'
The Observatory and Department of Civil Protection have put in place a two stage warning. In the first 'Attention' phase, local government puts heads of the communes, police, army and other emergency services on stand-by. 'Pre-alarm' is a declaration of a state of emergency precipitating evacuation.
'Earthquakes and ground deformation take place over the course of a few months before an eruption, which theoretically gives us warning, ' says Civetta.
Vesuvius is monitored constantly with GPS, gravitometric, EDM, tiltmetric, geochemical, seismic and radio sensors. Total stations are trained on its slopes and tide gauges are stationed along the shore to check for ground level changes.
To avoid overloading slender transport corridors it is imperative that evacuation is not a last minute rush, Civetta notes. But declaring a state of emergency more than a week in advance does create significant risk of a false alarm.
The local economy is driven by agriculture and tourism, industries that can ill afford to lose a week or two of production and revenue. If the area is cleared and no eruption takes place, pacifying the locals and winning back trust in the evacuation strategy will be as difficult as triumphing over the volcano itself, Civetta concludes.