One of the world's most dangerous supervolcanoes may erupt sooner than previously thought and threaten the safety of millions, scientists say.

In a study published Monday in Nature Communications, scientists from the University College London (UCL) and the Vesuvius Observatory in Naples, Italy, created a new model that looked at activity and ground uplift since the 1950s at the Phlegraean Fields supervolcano . Known locally as the Campi Flegrei caldera, the huge 8-mile-wide volcanic field lies just 9 miles west of the city that is home to more than 3 million people.

The scientists determined that a “long-term accumulation of stress” provides “the first quantitative evidence that Campi Flegrei is evolving towards conditions more favorable to eruption.”

According to Volcano Discovery, Campi Flegrei is comprised of 24 craters that are easily visible from space, with much of the field lying beneath the gulf of Pozzuoli. It last erupted in 1538, producing what is known as a "super-colossal" eruption that lasted more than a week. It was considered a rather small eruption considering the caldera's violent past.

While it is impossible to predict when a volcano will erupt, certain signs can help scientists predict when an eruption might occur.

Over the course of the past nearly 70 years, three episodes of activity in the '50s, '70s and '80s were recorded that involved the movement of magma 1.8 miles beneath the surface, accompanied by small earthquakes that created uplift at the caldera.

The scientists say the relatively recent activity is stretching the ground to the point where magma may eventually break through the crust. Previously, researchers believed the crust of the earth relaxed after each episode of activity, but this latest model proves otherwise.

In a statement , study author Christopher Kilburn, director of the UCL Hazard Centre, said based on their observations of the "cracking and moving at Campi Flegrei, we think it may be approaching a critical stage where further unrest will increase the possibility of an eruption, and it's imperative that the authorities are prepared for this.

"We don't know when or if this long-term unrest will lead to an eruption, but Campi Flegrei is following a trend we've seen when testing our model on other volcanoes, including Rabaul in Papua New Guinea, El Hierro in the Canary Islands, and Soufriere Hills on Montserrat in the Caribbean."

(MORE: Italian Supervolcano Under Naples Is Waking Up )

The scientists note that the uplift at Campi Flegrei will reach 16 to 33 feet before an eruption is likely to occur.

Kilburn told Newsweek that the biggest volcano eruption that theoretically could occur would equal that of the famous eruption of Mount Vesuvius in 79 AD, which destroyed Pompeii and Herculaneum.

“It’s much more likely to be about 100 times smaller," he told the news magazine. "The real problem is not knowing where it’s going to happen … We’ll have to evacuate more than the area that’s likely to be affected because they won’t know quite where it’s going to come out until the last moment, whereas a volcano like Vesuvius the magma normally comes out of the top.”

Kilburn made it clear that they are not saying that there will be an eruption in the near future, but that the activity makes the scenario more likely.

"There’s a big difference between ‘there will be one’ and ‘it’s more likely than previously thought,'" he told Newsweek. "If you evaluate the probability of it occurring, you have to understand the crust is becoming increasingly broken up, so the chances are increasing with time.”

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