(thoughtful music) - There's a place in the Caribbean where an entire city vanished.

Today, buildings still stand, but no one lives here.

Streets still stretch for miles, but no one walks them.

Because beneath these green hills simmers a devastating and destructive force that buried a town alive.

But what happened here isn't over.

Scientists say that this lost city and the forces that consumed it still have secrets to reveal.

(thoughtful music) (upbeat calypso music) For generations, Montserrat was a lush and lively island.

It's a place where calypso stars were born and rock and roll legends came to record, a place where locals and tourists alike soaked in the Caribbean sun.

But underground something was building.

It had been quiet for centuries, until one day it wasn't.

- July 18th, 1995, it was a regular Tuesday.

In the evening, I think most people started hearing what sounded like a jet engine.

(volcano rumbling) We couldn't really figure out what was happening.

Later in the evening, certain parts of the island started to get volcanic ash falling on them.

I mean, none of us knew what that was.

- At the beginning of the eruption in 1995, people were, like, saying that maybe the volcano would last maybe just two, three years as a lot of other volcanoes in the region.

- [Joe] But this was the beginning of a 15-year period of intense volcanic activity that dramatically reshaped the island.

- The major hazard from this volcano would be what we call pyroclastic flows.

Basically what it is is an avalanche of hot rock and ash with a hot gas cloud spreading out from that avalanche.

- [Joe] Pyroclastic flows like this can hurtle downhill at up to hundreds of miles per hour.

- [Thomas] When a flow destroys an area, because it's coming at you with boulders, it's gonna knock down walls or bury whatever it doesn't knock over.

- [Joe] For the residents, there was no fighting this.

- These flows, they would smash and bury everything in their path, so Plymouth was totally destroyed.

It just looks like a moonscape.

- [Joe] Homes, schools, businesses buried beneath a thick layer of volcanic debris.

- My parents basically lost their home.

My father had a business in Plymouth that was lost.

People basically lost everything and then had to kind of start again.

- [Joe] Two-thirds of Montserrat's 11,000 residents were displaced, scattered across the world.

Most of the island was swallowed by ash and fire.

Today, Plymouth remains still an empty capital, entombed in ash, frozen in time.

(car rumbling) - This is actually the original road.

What you are actually standing on here is buildings that are covered from the flow coming from the volcano itself.

There was actually a car park just beyond that area.

We used to play basketball in this area right there.

And then the cinema was just opposite the car park itself right there.

This building, if I memory serve me correctly, was like a supermarket area.

This is St. Anthony's Anglican Church right there.

That's the pulpit right there.

And then you have the pipe organ right over there.

I used to work mainly at the back, right behind there.

And this is where I witnessed my first blackout, on a Monday morning, just after eight o'clock, right?

Blackout is when there's an eruption, goes up in the air, block the sun.

Day turns into night.

And if I'm close, I cannot even see my hand because of the darkness.

And I actually witnessed three of those.

It was kind of horrifying to see that darkness turning to day.

- The volcano that buried Plymouth is still active today.

This destruction is part of a much bigger story, one that stretches across the entire Caribbean.

- The entire East Caribbean, which is from Grenada up to Saba, all of these islands were created by volcanic eruptions.

- So we're in the Caribbean on the Caribbean Plate, and the Atlantic Plate is butting into it and subducting beneath it.

- As the crust sinks, it melts, sending magma rising to the surface.

It's a slow process, but over millions of years, eruptions built this entire island chain, one volcano at a time.

Some of these volcanoes are long extinct, like the volcano which form the island of Antigua.

Others, like Soufriere Hills on Montserrat, may look dormant, but are still very much awake.

The big question is, will this happen again?

This is the Montserrat Volcano Observatory.

Every day here for the past 30 years, scientists have been looking for clues that might help them predict violent eruptions.

- I was 19 years in 1995 when it started.

I definitely did not know people got paid to work on volcanoes until the scientists came here and I started to speak to one or two of them and try to wrap my head around what the job was like.

We look at different parts of the system similar to a patient being looked after by different medical specialists.

You might have someone who's looking after his heart, somebody's looking at his lungs, somebody's looking at his liver.

For me personally, I'm looking at what's happening in the magma chamber.

(majestic music) (helicopter whirring) - [Joe] Strangely, to understand what's happening deep down in a volcano, sometimes you have to look up in the sky.

- [Thomas] The beauty of volcanic gases is that they get to the surface quicker than the magma would.

If we can analyze the plume composition of a volcano, we can say a lot about how deep the magma is, which is quite important.

We will be able to track the movement of the magma towards the surface.

- [Joe] And one gas tells that story better than almost any other: sulfur dioxide.

Dr. Thomas Christopher uses an ultraviolet spectrometer to measure how much of it is being released.

The spectrometer connects to a telescope mounted outside the helicopter pointing up into the volcanic plume.

- So what we do is we attach this spectrometer to the computer, and then we have a fiber optic cable attached to a telescope, which puts light into the spectrometer.

If I'm seeing a lot more sulfur dioxide, then I know that system is probably in eruption or getting ready to erupt, because sulfur dioxide in excess means that the system is hot, the magma chamber is hot.

- [Joe] Other scientists are looking for movement of the land, a sign that an eruption may be building.

- We monitor the formation around the volcano.

We use several methods.

So we can use what we call a EDM, electronic distance measurement.

The EDM is going to calculate distances between one point and another point on the volcano.

- We set up our tripod and our total station.

Then we shoot it to some reflectors we have around the volcano.

So we have these prisms all around the volcano.

We send the laser beam to that, bounces back and give us the exact distance between the benchmark and that reflector.

- [Karen] At the moment, we are seeing a continuous inflation of the volcano.

It's maximum about one centimeter per year horizontally.

So it's not very big and you can't detect it with your eyes, which means that there is a continuous pressurization of the system, and likely, given all the research that we have been doing, it seems that it would be some accumulation of magma.

So that's one of the criteria that indicates that the volcano is still active for sure and that it still has a potential to erupt.

- [Joe] No single measurement tells the full story, but when you put them together, you start to see the bigger picture.

- I don't think, like, personally that we can know everything which is happening inside the volcano, just because there are a lot of uncertainties.

- It's a very difficult process, 'cause the magmatic system is several kilometers beneath the surface and you're trying to understand what it's doing from the surface.

- [Joe] Scientists are also looking at clues like seismic activity.

- I'm the volcano seismologist.

I study earthquakes associated with the volcano, because as the volcano grows, there are lots of stresses happening underneath the volcano.

There is a standard sequence that you might expect in the buildup to an eruption.

We start off with the volcano tectonic earthquakes, the VT earthquakes.

That's basically the rock's being fractured, it's being stressed by the magma being injected somewhere.

Then these would transition into the low-frequency earthquakes, and these would start occurring in swarms, and they are happening 'cause the magma is actually moving fairly freely.

But that really indicates that the magma is on its way and is coming out and nothing's going to stop it.

(volcano booming) - [Joe] The final clue scientists look for?

Heat.

If magma is truly on the move, we should see rising temperatures.

(thoughtful music) (helicopter whirring) - I use what's called a FLIR camera or an infrared camera.

And now, this takes pictures of the lava dome, of the fumaroles.

Much like an ordinary digital camera does, but instead of looking at light, it's using infrared radiation or the heat being generated by the fumaroles to create the image.

Fumaroles are basically a vent in the surface of the Earth through which volcanic gases escape.

These fumaroles can range in temperature from less than 50 degrees to more than 500 degrees Celsius.

You clearly have to have a lot of heat down below to, you know, produce that high temperature at the surface.

With the fact that we've got fumaroles, you know, in excess of or around 500 degrees Celsius still 15 years after the last time that lava was erupted clearly indicates that there's a lot of heat still in the system below ground.

- [Joe] Together, these observations point to one conclusion: this volcano is still alive.

- [Adam] Right now, the biggest question that we have is, is the volcano going to carry on in this sort of long-term period of unrest, and how long is that going to last?

- We do what we can with the data we have, but there is a lot of uncertainty in this data and in the models.

They just show us that, well, like a good thing to do is just to keep on monitoring the volcano, basically, and to be maybe a little bit modest in our predictions.

(chuckles) - Although the city of Plymouth stands frozen in time, the island of Montserrat is very much alive.

The community rebuilt itself on the north half of the island.

But rebuilding hasn't just meant starting over; it's meant adapting, learning to live alongside the volcano.

The volcanic debris that once destroyed this island has become its most valuable export.

Montserrat ships construction-grade sand across the Caribbean.

People think of volcanoes as disasters, but in Montserrat here, they're proving that they can be resources too.

Montserrat is far more than just the buried city of Plymouth.

It's a spirited community.

(upbeat calypso music) ♪ Who's gonna give me ♪ ♪ Who's gonna give me ♪ ♪ Who's gonna give me ♪ ♪ Who's gonna give me ♪ ♪ Who's gonna give me ♪ - And for the people who live here, the volcano isn't just a threat; it's part of their story.

- The most consequential question is, is it going to erupt again?

If something is going to happen, there's a high probability that we will see it.

- [Thomas] You realize that this job is about protecting the people from the volcano and a geohazard that we might encounter, and we need to be good communicators.

(cheerful calypso music)