"Mount Vesuvius is currently the most threatening volcano in the world, because of the dense population living in the area," Franco Barberi, of Rome University, told AFP at a geology congress in the northern city of Florence.

"The risk is particularly high in the case of a high-intensity eruption, projecting clouds loaded with magma and incandescent matter," he explained.

Some 700,000 people live in the area of the Vesuvius, near the southern city of Naples which itself has a population of one million.

The volcano was made legendary by an eruption in the year 79, which engulfed the ancient cities of Pompeii and Herculanum.

Before the volcano's last major eruption, in 1631, 17th-century documents suggest that warning signals -- ground tremors, dense smoke -- had started appearing for several weeks.

Yet although geologists are increasingly able to predict when and where an eruption will take place and how far the ashes or lava will travel, regions such as Naples remain in danger.

The area can be divided into three, with varying degrees of risk for the population -- the most exposed zone, home to some 560,000 people, would have to be evacuated before the start of an eruption.

"It is a question of hours," in the case of a so-called pyroclastic eruption, involving a mixture of gas-rich magma, water vapour and debris, flung from the volcano at extremely high temperature, Barberi warned.

Italian authorities have made plans for a mass evacuation, with every region in Italy having agreed to provide for a town in the Vesuvius area, and simulated evacuations have been staged.

The region of Campania of which Naples is the capital has also adopted a 700-million-euro (840-million-dollar) plan to stop further construction in the highest-risk area, with stiff penalties for offenders, and cash incentives for people willing to relocate outside the area.

Using geothermal and geophysical tools, scientists are generally able to predict an eruption and trigger an evacuation if necessary: in the Philippines, the Pinatubo eruption in 1991 claimed 300 lives, compared to up to 80,000 had the area not been evacuated.

Scientists have used Mount Etna in Sicily, which has had regular bursts of activity over the past few decades, as an open-sky laboratory to study lava flows and learn to slow and channel them away from human habitations.

In most cases, lava flows extremely slowly and poses little threat to humans, although the Nyiragongo in the Democratic Republic of Congo is an exception -- its last eruption in 1982-1983 sent lava hurtling at 60 kilometres per hour (35 miles per hour) to the outskirts of the eastern town of Goma.

According to the Smithsonian Institute, 1,415 volcanos have erupted in the past 10,000 years, with the most active being Mount Etna, Stromboli off the coast of southern Italy, and the volcanoes of Hawaii.

All rights reserved. � 2005 Agence France-Presse. Sections of the information displayed on this page (dispatches, photographs, logos) are protected by intellectual property rights owned by Agence France-Presse. As a consequence, you may not copy, reproduce, modify, transmit, publish, display or in any way commercially exploit any of the content of this section without the prior written consent of Agence France-Presse.