The slumbering Campi Flegrei volcano under the Italian city of Naples shows signs of 'reawakening', a new study has warned.

Researchers said that the simmering volcano may be nearing a critical pressure point.

For the first time, they have identified a threshold beyond which rising magma under the Earth's surface could trigger the release of fluids and gases at a 10-fold increased rate.

This could potentially lead to an eruption, which could be 'very dangerous' for the half million people living in the area.

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The slumbering Campi Flegrei volcano under the Italian city of Naples shows signs of 'reawakening', a new study has warned. The Solfatara of Pozzuoli (pictured) is one of forty volcanoes that make up the caldera of the supervolcano

CAMPI FLEGREI The Campi Flegrei crater was formed 39,000 years ago in a blast that threw hundreds of cubic kilometres of lava, rock and debris into the air. It was the largest eruption in Europe in the past 200,000 years, according to scientists. Campi Flegrei last erupted in 1538, though on a much smaller scale. Nearby Mount Vesuvius, whose massive eruption just over 2,000 years ago buried several Roman settlements in the area, including Pompeii, is also classified as an active volcano. Advertisement

An increase in the release of fluid and gases would cause the injection of high-temperature steam into surrounding rocks, said lead author Giovanni Chiodini, a researcher at Italy's National Institute of Geophysics and Volcanology in Bologna.

'Hydrothermal rocks, if heated, can ultimately lose their mechanical resistance, causing an acceleration towards critical conditions,' he told AFP.

It is not possible at this time to say when - or if - the volcano will erupt anew, he said.

If it did, however, 'it would be very dangerous' for the half-million people living inside and near the caldera, he added, using the scientific name for the bowl-like depression created after a volcano blows its top.

Since 2005, Campi Flegrei has been undergoing what scientists call 'uplift', causing Italian authorities to raise the alert level in 2012 from green to yellow, signalling the need for active scientific monitoring.

The pace of ground deformation and low-level seismic activity has recently increased.

The Campi Flegrei is located underneath the Italian city of Naples and could prove dangerous for the half million people living within range if it were to erupt

Two other active volcanoes - Rabaul in Papua New Guinea, and Sierra Negra in the Galapagos - 'both showed acceleration in ground deformation before eruption with a pattern similar to that observed at Campi Flegrei,' Chiodini said.

The Campi Flegrei caldera was formed 39,000 years ago in a blast that threw hundreds of cubic kilometres of lava, rock and debris into the air.

It was the largest eruption in Europe in the past 200,000 years, according to scientists.

The caldera of the Campi Flegrei is studded with the craters of past eruptions and volcanic vents (pictured). Far beneath the surface magma is building up and creating huge pressures

An eruption of the Camp Flegrei supervolanco would dwarf the devastation caused by Mount Vesuvius on the otherside of Naples (illustrated)

Campi Flegrei last erupted in 1538, though on a much smaller scale.

Nearby Mount Vesuvius, whose massive eruption just over 2,000 years ago buried several Roman settlements in the area, including Pompeii, is also classified as an active volcano.

The dense urban population at risk 'highlights the urgency of obtaining a better understanding of Campi Flegrei's behaviour,' Chiodini said.

The study was published in the journal Nature Communications.