Campi Flegrei, Italy’s supervolcano, is one of the most dangerous in the world, not least because over 1 million people live in the barrel of its gun.

Now, a new study in Scientific Reports may have identified the source of the magma that’s fueling the dormant and ominous cauldron – and worryingly, things look even more dangerous that anyone’s truly appreciated.

Normally, scientists can use seismic waves emitted from magma forcing its way through the crust to identify where the magma actually is with considerable accuracy. This supervolcano has been generally quiet since the mid-1980s though, which means that it’s been far more difficult to locate its molten horrors.

An international team, led by the University of Aberdeen, decided to attempt to solve this enigma. Using a specialized type of mathematical analysis on seismic data collected from the mid-1980s, the team located a hot zone around 4 kilometers (around 2.5 miles) beneath Pozzuoli, a town near Naples.

According to the study, this is either a small batch of magma or it’s the molten top of a massive magma chamber that’s spread its liquid fire deep beneath the surface. Either way, it’s strong evidence that there is an active heat source contributing to a magma supply to one of the world’s most dangerous volcanoes – but the story doesn’t end there.

One of the key mysteries about Campi Flegrei is its periodic and frightening inflation. Between 1982 and 1984, the ground within the crater rose by 1.8 meters (5.9 feet). Whatever the cause – magma rising, gas moving through the crust, or superheated water migration – it deflated soon after that without making so much as a stir.

This new study offers up an explanation as to why this inflation didn’t culminate in a volcanic eruption; namely, based on the seismic imaging, a particularly rigid shallow rock formation above the magma prevented it from breaching the surface. Instead, it was spread out laterally and failed to erupt.

This means that “the risk from the caldera… has migrated. You can now characterize Campi Flegrei as being like a boiling pot of soup beneath the surface,” lead author Dr Luca De Siena, a geoscientist at Aberdeen, said in a statement.

Instead of a single eruptive point, a vast swath of the entire region may explode at some point down the line.

Campi Flegrei remains a very poorly understood monster. It first formed 40,000 years ago during one of the most energetic paroxysms of the last few million years; up to 500 cubic kilometers (120 cubic miles) of debris was generated, some of which landed in Greenland 4,600 kilometers (2,860 miles) away.

Since then, it’s had a few violent eruptions, but it has left most of the fireworks to several volcanoes located near or within the crater itself, such as Mount Vesuvius and the sinister, sulfuric Solfatara. Volcanologists remain acutely aware of its risks to the 6 million people residing in the beast’s “blast zone,” and as such, are continuously monitoring it closely.

Something that’s been a cause for concern is that Campi Flegrei is inflating once again, but the rate is 24 times lower than that of the early 1980s. As ever, volcanologists cannot be sure what’s going on, but a consensus is building that the volcano is trending towards a critical point, past which an eruption is inevitable.

Whether or not that will be a caldera-forming supereruption or just a “regular” violent one remains to be seen. Either way, according to De Siena, “there is no doubt that the volcano is becoming more dangerous.”

Forget Yellowstone. This is the supervolcano you should really be keeping an eye on.