Ancient supervolcano that caused the largest eruption in European history is stirring again

The Italian name for the caldera - Campi Flegrei, or “burning fields”- is apt.

The 12km-wide cauldron is the collapsed top of an ancient volcano, formed when the magma within finally blew under what is now the city of Naples. Though half of it is obscured beneath the crystal blue waters of the Mediterranean, the other half is studded with cinder cones and calderas from smaller eruptions. And the whole area seethes with hydrothermal activity: Sulphuric acid spews from active fumaroles; geysers spout water and steam and the ground froths with boiling mud; and earthquake swarms shudder through the region, 200km south of Rome.

And things seem to be heating up. Writing in the journal Nature on Tuesday, scientists report that the caldera is nearing a critical point at which decreased pressure on rising magma triggers a runaway release of gas and fluid, potentially leading to an eruption.

Forecasting volcanic eruptions is a famously dicey endeavour, and right now, it’s impossible to say if and when Campi Flegrei might erupt, according to lead author Giovanni Chiodini, a volcanologist at the National Institute of Geophysics in Rome. But now more than ever, the caldera demands attention: An eruption would be devastating to the hundreds of thousands of people living in and around it.

The site’s last major eruption happened over the course of a week in 1538, when it expelled enough new material to create the cinder cone mountain Monte Nuovo.

But the caldera itself is some 39,000 years old, formed by an eruption larger than anything else in the past 200,000 years of European history. A 2010 study in the journal Current Anthropology suggested that this prehistoric outburst - which spewed almost 4 trillion litres of molten rock and released just as much sulphur into the atmosphere - set off a “volcanic winter” that led to the demise of the Neanderthals, who died out shortly afterward.

Today, the Campi Flegrei caldera is increasingly restless. For half a century, scientists have measured “bradyseism” events - slow movements of the ground - that are indicative of molten rock slowly filling the mountain’s magma chamber. Significant uplift in the past decade prompted Italian authorities to raise the supervolcano’s alert level from green (quiet) to yellow (scientific attention) in 2012.

“These areas can give rise to the only eruptions that can have global catastrophic effects comparable to major meteorite impacts,” Giuseppe De Natale, head of a drilling project to monitor the caldera, told Reuters after that change was made in 2012.