NOVA

In Southern Italy, a city under threat:

Naples, flanked by two dangerous volcanoes.

On one side, Vesuvius,

destroyer of ancient Pompeii.

None of them met with a peaceful death.

They were afraid, they were panicked,

and it was terrifying.

And on the other, Campi Flegrei,

an invisible monster

that threatens three million people.

This is the most dangerous volcano in the world.

Now scientists are seeing increases in volcanic activity

and are scrambling to unlock the inner secrets

of these volcanoes, before it's too late.

There's no choice.

You just have to go to a place

which is relatively difficult to get to.

Can they find ways to predict the next eruption?

This is evidence of how fast things change in this area.

Can they save this jewel of Italy

from another disaster?

This volcano can erupt at any time...

Even tomorrow.

Could Naples become the next Pompeii?

Right now, on "NOVA."

Major funding for "NOVA" is provided by the following:

More than 1,000 volcanoes around the world are active.

Driven by Earth's fiery interior,

they can erupt at any time,

blasting molten rock and ash into the sky

with frightening power and speed.

And unleashing deadly destruction

on those who live in their shadow.

Volcanoes have the power to kill thousands

in the blink of an eye.

A stark warning for the people who live here,

in the beautiful port city of Naples in Southern Italy.

This ancient and vibrant metropolis

of over three million people

might seem an ideal place to call home.

But looks can be deceiving.

It is built right next to not one,

but two active volcanoes.

Each has a history of catastrophic eruptions.

To the east is the well-known Vesuvius,

a classic, cone-shaped volcano

that has claimed thousands of lives,

erupting as recently as 1944.

And to the west is an almost unknown volcano,

Campi Flegrei, hidden below ground

in an area where hundreds of thousands live.

It doesn't even look like a volcano.

And yet, it has the potential to be far more destructive

than its more famous neighbor.

Today, there are ominous signs

that both volcanoes are still active:

swirling clouds of gas and bubbling pools of mud.

What do these mean about the likelihood

of a major eruption?

It's hard to imagine the impact

on such a densely populated city.

And yet, it's happened before.

Just 15 miles from central Naples

is the site of one of history's most infamous

volcanic disasters.

The Roman city of Pompeii.

In year 79 of the Common Era...

Vesuvius exploded and buried the entire city

in over a dozen feet of ash.

Over the last 150 years,

archaeologists have been carefully

uncovering the remains...

Revealing a scene of carnage and death.

A city and its people, frozen in time

at the exact moment an eruption struck.

Among these ruins are crucial lessons

about how a volcanic disaster can unfold

and evidence of just how suddenly an eruption can strike.

For archaeologist Kevin Dicus,

this human tragedy is a stark warning from history.

Walking around the ruins today, it's really easy to forget

that this was actually a functioning Roman town

2,000 years ago.

Within the city walls, perhaps 15,000 people

of different classes, different ethnic makeup,

all trying to survive in this city.

Remarkably, everyday items, like food,

survived the eruption.

Walnuts.

Walnut kernels, already peeled.

One of these food items shows how the people of Pompeii

were caught completely off-guard when Vesuvius erupted.

This one is one of the 81 breads

found inside an oven that was fully operational

- at the moment of the eruption. - Wow.

With one peculiarity.

There is still the fingerprint from the baker,

the baker's thumb.

Oh, my gosh.

This really brings a human element to this, right?

Bread being baked at the time of the eruption,

and the baker... hopefully the baker escaped,

hopefully he lived the rest of his days

baking bread somewhere else, but we have no idea.

But it really it tells us something about the eruption,

how unexpected it was.

In the face of this sudden cataclysm,

many of Pompeii's residents fled in blind panic.

But some chose to stay behind and shelter in the city.

They paid the ultimate price.

Their last desperate moments preserved for eternity,

as Pompeii's most evocative remains.

These are but a few

of the approximately 2,000 victims

that decided to wait out the eruption

and, of course, didn't make it.

Now, what we have here are not the bodies themselves,

but these are the exact poses of the victims

at the very last moments of their lives.

After their death, they were covered with ash.

Over time, the soft tissue decayed, liquefied,

and leaked through the ash, leaving behind these voids.

Archaeologists pour in plaster of Paris,

leave them to dry, and we get these exact poses.

This is a really macabre reminder

of what can happen in the blink of an eye

to an entire city.

Could Vesuvius erupt today with such violence?

Do millions of people risk the same fate

as their ancestors?

Centuries have passed, but one thing hasn't changed.

Vesuvius is still active.

Driven by a geological collision

that has been shaping this part of Italy for millions of years.

To the east of Naples,

two of the planet's vast tectonic plates

are crashing into each other.

The African plate is being forced downwards

in a process called subduction.

As it descends toward the hot center of the Earth,

it gets warmer.

This causes rocks to melt and turn into liquid magma,

which rises towards the surface.

To the east of Naples,

there are weaknesses in the rock.

Here the magma can break through.

And if it has enough power,

it can even trigger an eruption.

It's this constant subduction which keeps Vesuvius active

and fuels its eruptions.

Due to this ever-present danger,

the volcano is kept under 24-hour surveillance.

At the Osservatorio Vesuviano,

scientists are watching for the warning signs

of an imminent eruption.

We have one millions and five hundred people

that are at high risk in case of eruption.

From a high-tech control center,

Francesca and her colleagues pick up seismic activity...

Movements in the Earth's crust.

In these screens, we see all the signals

that came from the sensors

that we have installed on Vesuvius.

In all, there are 150 sensors on the volcano,

all listening for earthquakes.

These can indicate if an eruption is on the way.

Each eruption begins when magma starts to rise upwards

from deep in the Earth.

It pushes through weaknesses in the rocks

towards the surface.

If its way is blocked,

it builds up more and more pressure

until it fractures the rocks apart.

This is so violent,

it generates earthquakes which the scientists can detect

at the surface.

Every time the magma breaks through the rock,

it generates a new quake

until the magma erupts out of the volcano.

Francesca and her colleagues detect minor earthquakes

nearly every week.

And this constant monitoring is vital.

Vesuvius can change very quickly.

We have active volcano, and of course,

if a volcano is active, for sure,

it will erupt again.

We don't know when.

Mindful of Vesuvius's lethal past,

the observatory's scientists don't rely solely

on earthquake monitoring.

They have another, more hands-on way

to monitor the magma.

At the summit of Mount Vesuvius is a vast crater,

nearly 1,000 feet deep.

Every month,

a monitoring team carefully descends into this gaping hole

to gather data.

Today, geologist Chris Jackson is joining them.

Some of the critical bits of data

about the volcano's behavior and the kind of threat it can pose,