There are 1.5 million people living inside an Italian volcano - and scientists have found it's building up for an eruption

Fact: 1.5 million lives are not in any danger from an imminent eruption in Italy.

Also fact: as many as 1.5 million people live inside the bounds of a caldera that stretches some 12km from the western edge of the city of Naples to the Mediterranean Sea.

The volcano is called Campi Flegrei and for tens of thousands of years, it's gone through the traditional cycle of growth and collapse, as volcanoes do.

Scientists know of at least two magma buildups under Campi Flegrei that have inevitably released and resulted in a caldera collapse - one 39,000 years ago, and another 15,000 years ago.

The first spewed out around 191 billion cubic metres of magma - roughly 800 times more than Hawaii's Kilauea produced in its most active month this year when it destroyed 650 homes on Big Island.

Worldwide, we know of just seven caldera collapses in a little over the past 100 years, so between that and the fact Campi Flegrei has erupted just twice in 39,000 years, hopefully a picture emerges of no need to be ringing any alarm bells.

And just in case, you can rest assured that Campi Flegrei is monitored around the clock for even the slightest hint of activity. It's next door volcanic neighbour is Vesuvius.

It is absolutely not going to erupt in any way that would surprise us, and there definitely are not 1.5 million lives in danger, okay?

Still.

In 1538, a Campi Flegrei eruption built a 122m high hill that became known as "Monte Nuovo". And research nowadays reveals that conditions before "new mountain" was formed were similar to those that preceded the big caldera-forming events.

A study of Monte Nuovo's formation published today in Science Advances reveals that those caldera-forming conditions might now be in play again on Campi Flegrei.

Magma is once again building up under the volcano, the team from ETH Zürich in Switzerland found, noting that "caldera-forming magmatic systems often follow recurrent evolutionary paths".

It's the first time someone has researched the "magmatic evolution" of a system, rather than single eruptions or limited periods of activity.

The point of it is not to predict an exact moment when Campi Flegrei will erupt, because that's impossible.

"Volcanos are not clocks, either in terms of time, or behavior," lead author and volcanologist Francesca Forni told The Verge.

But "generally" she said, "magmas want to erupt."

What the study does is add a new set of data to that being built about how Campi Flegrei - and other volcanic regions - behave.

You can read more about how Forni's team came to its conclusion at National Geographic here, but the short version is they studied volcanic material from 23 eruptions.

Modern techniques allowed them to identify certain characteristics about how the material formed, and that data was fed into a computer that in turn simulated all of Campi Flegrei's eruptions since the last big one 15,000 years ago.

What they found was evidence that magma went through cycles of cooling and heating. According to Forni, the chemistry of the last eruption, which resulted in Monte Nuovo, "indicate(s) that the magmatic reservoir might be 'ready' for accommodating magmas from recharge without erupting frequently".

The team also noted that recent activity, such as gas emissions and ground deformation, could be a sign that magma could be on the move.

Or not. Volcanologists are quick to point out the dangers of linking short-term changes to activity that historically occurs in cycles counted in tens of thousands of years.

Forni told NatGeo that "we actually don’t know for sure what the next step is going to be". Just that there may be a pattern to changes in magma formation that leads to caldera collapse - and that's something we didn't know a lot about before.

You can read more about the study here.