- [Narrator] Vesuvius isn't the biggest or most powerful volcano in town.

On the other side of the city, scientists have seen an alarming increase in volcanic activity.

(soft dramatic music) Pisciarelli, a vast fumarole, a vent of bubbling gas and mud.

(volcanic mud bubbling) In recent years, it has grown larger, and turned into a destroyer.

(ominous music) A few hundred feet away, expert in volcanic risk, Antonio Acosta, is visiting a deserted building.

(ominous music) Inside, the walls and floors are covered in a thick layer of solidified ooze.

(ominous music) And the air is filled with the acrid smell of sulfur.

(ominous music) The volcanic vent has claimed the entire building.

This hostile takeover, began when gas from the vent punched holes through the floor and walls.

(ominous music) - Here you can even feel, if you put the hand here, ah, it's terribly hot.

(ominous music) - [Narrator] As the gas cools, it releases dissolved minerals.

(ominous music) These stick together to form the thick volcanic scum, which is now consuming the entire house.

(Volcanic mud bubbling) So where is the volcano that is driving all this activity?

The vent is in an area called Campi Flegrei, which is home to hundreds of thousands of people.

(gentle music) At first glance, it's hard to spot the volcano, (gentle music) unless you have a trained eye.

- Good?

- Yeah.

- [Narrator] To take a better look at the lay of the land, Chris needs to take to the air.

A monitor connected to a drone, provides him with a bird's eye view of Campi Flegrei.

(drone buzzing) (ominous music) - So the first thing you notice from up in the air is, as we come round to the shoreline towards the Bay of Naples, we start to pick up a prominent ridge line.

It comes all the way round towards the main city center.

It's clearly curved.

And then within there, it's a very densely populated flat area here.

This ridge line, could be telling us something about a feature that is actually forming this landscape.

An ancient, very large volcano, and in particular, we're actually looking at a Caldera.

- [Narrator] A Caldera is a collapsed volcano.

(ominous music) In the past, Campi Flegrei was a flat plain.

(ominous music) Deep beneath was a huge reservoir of bubbling magma.

(ominous music) (magma rumbling) Then the magma started moving upwards.

(magma rumbling) Smashing through weaknesses in the rock.

(magma rumbling) (dramatic music) and erupting powerfully.

(dramatic music) (magma rumbling) But this left an empty void beneath the surface.

(dramatic music) (magma rumbling) (dramatic music) With nothing left to support the weight of the plain, it collapsed downwards.

(ground rumbles) (dramatic music) Forming a crater, known as a Caldera.

(dramatic music) Radioactive dating of rocks reveals that Campi Flegrei's Caldera, formed 15,000 years ago.

(somber music) And at nearly eight miles wide, it must have been created by an incredibly large and powerful eruption.

(ground rumbling) (somber music)