Campi Flegrei, Europe’s only supervolcano, is showing signs of reawakening and its eruption could affect half a million people, according to a new study. Magma could trigger the release of fluids and gases at an increased rate after a certain threshold, the researchers said.

“Of the several quiescent calderas worldwide, Campi Flegrei has recently shown among the clearest signs of unrest,” the study by Italian and French scientists from the National Institute of Geophysics and Volcanology in Bologna, Italy, noted.

The study was published Tuesday in the journal Nature Communications.

“Hydrothermal rocks, if heated, can ultimately lose their mechanical resistance, causing acceleration towards critical conditions,” lead author Giovanni Chiodini told Agence France-Presse. The population at risk “highlights the urgency of obtaining a better understanding of Campi Flegrei's behavior,” Chiodini noted.

It, however, remains unclear when exactly the volcano will erupt. Scientists have been detecting an increase of low-level activity and heating since 2005 in the area. They also have been noticing ground and magma deformation in the area. In 2012, the status of Campi Flegrei was changed from green, which refers to the volcano being quiet, to yellow, which means it needs scientific attention.

“The presence of more than half a million people living in the proximity of the [Campi Flegrei] caldera makes this situation particularly challenging for local authorities and other decision-makers,” according to the research.

Campi Flegrei was formed nearly 40,000 years ago in an explosion that spewed out several cubic miles of magma, rock and lava. The supervolcano last erupted in 1538. The eruption was relatively smaller and created a new hill called Monte Nuovo.

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