The volcanic eruption that formed the Campi Flegrei caldera about 39,000 years ago is believed to be the largest eruption in Europe in the past 200,000 years.

The supervolcano on the coast of Italy last erupted in 1538. Now, the 7.5-mile-wide cauldron-like depression of the volcano shows signs of stirring up again after nearly 500 years of inactivity.

Sleeping Supervolcano Shows Signs Of Reawakening

In a new study published on Dec. 20, researchers reported that the slumbering volcano under the city of Naples in Italy shows signs of "reawakening" and that it may be near a critical pressure point.

Over the past decade, the volcano experienced what scientists call uplift, a phenomenon that suggests the volatile gases under the volcano are rising to the surface at an accelerating rate. The significant uplift has prompted Italian authorities to raise the alert level of the supervolcano from green (quiet) to yellow (scientific attention) four years ago.

In the new study, which was published in the journal Nature Communications, study researcher Giovanni Chiodini, from Italy's National Institute of Geophysics and Volcanology in Bologna, and colleagues have identified a threshold beyond which the rising magma below the surface may set off the release of fluids and gases at a significantly increased rate. Chiodini said that this may expose the surrounding rocks to high-temperature steam.

"The abrupt and voluminous release of H2O-rich magmatic gases can heat hydrothermal fluids and rocks, triggering an accelerating deformation that can ultimately culminate in rock failure and eruption," Chiodini and his colleagues wrote in their study.

Is The Supervolcano About To Erupt Again?

The researchers, however, said that their observations do not necessarily indicate that the supervolcano is about to blow up. Chiodini said that it is not possible to say when or if the supervolcano could erupt.

"We have many uncertainties and long-term previsions are at the moment not possible! For example, the process that we describe could evolve in both directions: toward pre-eruptive conditions or to the finish of the volcanic unrest," Chiodini said.

If an eruption does happen, however, the event could prove catastrophic. The eruption that took place thousands of years ago triggered a volcanic winter that led to the demise of the Neanderthals.

About 500,000 people currently live inside and near the caldera. The researchers said that given the risk posed to a dense urban population, it is necessary to get a better understanding of the volcano's behavior.