A long-quiet yet huge supervolcano that lies under 500,000 people in Italy may be waking up and approaching a "critical state," scientists report this week in the journal Nature Communications.

Based on physical measurements and computer modeling, "we propose that magma could be approaching the CDP [critical degassing pressure] at Campi Flegrei, a volcano in the metropolitan area of Naples, one of the most densely inhabited areas in the world, and where accelerating deformation and heating are currently being observed," wrote the scientists—who are led by Giovanni Chiodini of the Italian National Institute of Geophysics in Rome.

A sudden release of hot magmatic gasses is possible in the near future, which could trigger a large eruption, the scientists warn. Yet the timing of any possible eruption is unknown and is currently not possible to predict.

In response to the news, Italy's government has raised the volcano's threat level from green to yellow, or from quiet to requires scientific monitoring. In other words, the government is urging a measured response to the study, followed by additional scientific work.

Campi Flegrei means "burning fields" in Italian. The volcanic region is also known as the Phlegraean Fields. Like other supervolcanoes—such as the one responsible for the geothermal features of Yellowstone—it is not a single volcanic cone. Rather, it's a large complex, much of it underground or under the Mediterranean Sea, that includes 24 craters, as well as various geysers and vents that can release hot gas.

Living Up Close and Personal With an Active Volcano

Supervolcanoes are usually characterized by a large caldera, or depression, that formed from past explosive eruptions. Campi Flegrei's depression, just west of Naples, is more than seven miles across.

Campi Flegrei is thought to have formed hundreds of thousands of years ago. A massive eruption 200,000 years ago spewed so much ash that it darkened the skies around the planet, triggering a "volcanic winter." That event is thought to have been the largest volcanic episode in the history of Europe over that time.

The volcano erupted again 35,000 and 12,000 years ago. An eruption about 40,000 years ago might have contributed to the extinction of the Neanderthals, a 2010 study suggested, although that report has been debated. The volcanic area was also known by the ancient Greeks. (See pictures of another supervolcano.)

A smaller but still sizable eruption was observed at the supervolcano in 1538. That event lasted eight days and created the mountain Monte Nuovo. But since then, the volcano has been quiet, slumbering for more than 500 years.

The scientists caution that it's possible nothing will happen in our lifetimes. They say it's impossible to say with any certainty when an eruption might actually take place. More monitoring and study are needed, they say.

Volcanoes 101

The scars of another supervolcano were recently found in the Sesia Valley in the Italian Alps. That eight-mile-wide caldera likely last erupted 280 million years ago, when it blasted out a thousand times more material than Mount St. Helens spewed during its infamous 1980 eruption. The result was the blocking out of the sun, which led to global cooling.

"There will be another supervolcano explosion," scientist James Quick, a geologist at Southern Methodist University in Texas, said in a statement when that volcano was found.