Forget Yellowstone. An active supervolcano in Italy appears to be edging ever closer to an eruption, with scientists warning it is like a “like a boiling pot of soup” simmering beneath the surface and that there is “no doubt that the volcano is becoming more dangerous.”

Campi Flegrei is a huge caldera—a crater that often forms after the mouth of a volcano has collapsed following an eruption—that sits to the west of Naples. Close to a million people live within the city’s administrative limits, making it one of the most densely populated areas in Europe.

The last time Campi Flegrei erupted was in 1538, but this was a relatively minor event. Around 40,000 years ago, it produced a “super colossal” eruption, ejecting an estimated 48 cubic miles of magma. This event has previously been linked with the extinction of the Neanderthals.

In the 1980s, the volcano started rumbling back to life. At this time, it produced a series of earthquakes under the city of Pozzuoli and the ground rose by 1.8m (6 feet), raising fears of an eruption. In a recent study, researchers showed how it had been building pressure for almost seven decades, and that this long-term accumulation of stress was evidence that Campi Flegrei is moving “towards conditions more favorable to eruption.”

To better understand what is going on beneath the volcano, a team of researchers led by Luca De Siena, from the University of Aberdeen, U.K., probed the caldera by analyzing the seismic swarms during the 1983–1984 unrest. Published in the journal Scientific Reports, the team was able to identify the first direct evidence of a hot zone that is feeding the supervolcano.

“One question that has puzzled scientists is where magma is located beneath the caldera, and our study provides the first evidence of a hot zone under the city of Pozzuoli that extends into the sea at a depth of 4 kilometers (2.4 miles),” De Siena said in a statement.

Findings indicate the magma did not rise all the way up to the surface during this period because of a rock formation that extends a mile deep beneath the ground blocking its path. Instead, the stress that was built up migrated toward a shallower region.

What this all means is not entirely clear, and the scientists say future research must focus on characterizing the features as we are at a point where the “geophysical and geochemical signals indicate that the volcano is reactivating.”

Discussing the findings, De Siena said that over the last three decades, the behavior of the volcano has changed and everything has become “hotter” because of fluids entering the caldera. “Whatever produced the activity under Pozzuoli in the 1980s has migrated somewhere else, so the danger doesn’t just lie in the same spot,” he said. “It could now be much nearer to Naples which is more densely populated. This means that the risk from the caldera is no longer just in the center, but has migrated.  Indeed, you can now characterise Campi Flegrei as being like a boiling pot of soup beneath the surface.

“What this means in terms of the scale of any future eruption we cannot say, but there is no doubt that the volcano is becoming more dangerous. The big question we have to answer now is if it is a big layer of magma that is rising to the surface, or something less worrying which could find its way to the surface out at sea.”