Everyone has heard of Vesuvius but the caldera of Campi Flegrei is a far more dangerous volcano.

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Campi Flegrei – the name means “fiery fields” – is a type of volcano known as a caldera (from the Spanish for “kettle” or “cauldron”). Other examples include Yellowstone in Wyoming, Rabaul in Papua New Guinea and Tennger in Indonesia. It was probably formed around 35,000 years ago when an immense prehistoric volcano erupted and collapsed inwards, creating a large bowl-shaped depression in the landscape. Twenty thousand years later a second eruption contorted the original caldera to give it its current shape: it is 15km across at its widest and extends beneath parts of Naples and under the Bay of Pozzuoli. Many smaller eruptions have shaken the caldera, the last of which occurred in September 1538. That eruption was fairly modest, though it still buried an entire village underneath a cone of earth 133 metres high and 700 metres across, known today as Monte Nuovo – the new mountain. Eyewitness accounts describe the ground swelling and cracking. Cold water gushed out, followed by bulging clouds of smoke and “deep-coloured flames”. Burning ashes and white-hot pumice were thrown 5.5km into the air and there was, according to an eyewitness, a “noise like the discharge of a number of great artillery”. So many birds fell dead from the sky that the ground around the eruption site was carpeted with their carcasses. Layers of ash and pumice, up to 25cm thick, covered buildings and vegetation.

For the next 400 years, the volcano dozed. The heat of the springs and the smoke from the fumaroles were the only signs of life. Most people’s attention turned to the more active and more dramatic cone of Vesuvius to the east of the city. Now, however, there are signs that Campi Flegrei is rousing. This is a major problem as the population in the area has swelled to over half a million people over the last 500 years, making it one of the most heavily populated calderas in the world.

“It’s much more dangerous than Vesuvius because we don’t know where the eruption will be,” said Morra. Unlike Vesuvius, where the eruption is likely to come from the top or side of the cone, a caldera has the potential to erupt in many different locations simultaneously. “But people are more scared of Vesuvius because with Campi Flegrei you don’t see the cone, so there is not the same perception of danger,” he said.

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