The Campi Flegrei (Phlegraean Fields) volcano may be less well-known than Vesuvius, but is "extremely dangerous", study co-author Stefano Carlino told AFP.
Vesuvius wiped Pompeii off the map almost two millennia ago, while the vast volcanic Campi Flegrei area near Naples last spewed lava, ashes and rocks in 1538.
But the Campi Flegrei is not one to take lightly -- the volcano's eruption 30,000 years ago is reported to have contributed to the extinction of Neanderthal man.
One could be forgiven for not realising its danger though, because instead of growing into a traditional mountain, the volcano has the shape of a gentle depression 12-14 kilometres (7.5-8.5 miles) across.
A resurgence of activity in the early 1980s led to the evacuation of 40,000 inhabitants, but the volcano has been relatively quiet since then.
"We're not saying there will be an eruption, we are saying that the conditions for a eruption are more favourable," Christopher Kilburn from University College London told AFP.
The tens of thousands of small earthquakes that have taken place since the 1950s weakened the caldera, the basin at the top of the volcano.
And the report -- published in Nature's Communications Earth & Environment journal -- found "parts of the volcano had been stretched nearly to breaking point".
The quakes have been increasing in number since 2019, while the pressure below has been building.
The coastal town of Pozzuoli has been lifted by nearly four metres (13 feet) -- roughly the height of a double-decker bus -- since the 1950s, it said.
The tremors and ground uplift are cumulative, meaning volcanic activity does not need to intensify for an eruption to become more likely.
"An eventual eruption could be preceded by relatively weak signals, such as a smaller rate of ground uplift and fewer earthquakes," the study's authors said in a press release.
They point to the eruption of the Rabaul caldera in Papua New Guinea 1994, which was preceded by small earthquakes occurring at a tenth of the rate than had occurred during a crisis a decade before.
- Small eruptions -
The probability of a massive eruption occurring, however, is "very low", Carlino said. "What is more likely are small eruptions."
And while the volcano is closer to rupture, that does not necessarily mean an eruption will take place, Kilburn said.
Even if the crust cracks, "the magma needs to be pushing up at the right location for the eruption to occur", he said.
The researchers used a model based on the physics of how rocks break and applied it in real-time to the volcano, which is flat and mostly hidden -- either under buildings or coastal waters.
They measured the tremors and ground movements and compared them with previous eruptions of other, similar volcanos.
"We cannot say with certainty what will happen, what matters is being prepared for any eventuality," Carlino said.
Some 500,000 people live in what Italy's civil protection agency has designated the red zone -- the area at highest risk.
Another 800,000 people live in the yellow zone.
Authorities have drawn up an evacuation plan, under which residents will be moved out using their own or public transport within three days.
The risk level -- green, yellow, orange and red -- is reviewed monthly.
"The alert level in Pozzuoli is currently yellow," council spokeswoman Giordana Mobilio told AFP, adding that locals receive alerts for all tremors of a magnitude of 1.5 or greater.
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