♪ ♪ NARRATOR: In the heart of Africa are two of the world's most dangerous volcanos.

Within their craters, molten lava steams and boils.

Over centuries these volcanoes have erupted many times, but when will they erupt again?

It's a crucial question, and no one knows the answer because these are among the least-studied volcanoes in the world.

♪ ♪ Now an international team of scientists has come to investigate these giants, to predict future eruptions and save lives.

BENOIT SMETS: Everything we do to understand this volcano is very important to avoid another disaster.

(helicopter droning) NARRATOR: Today, they're flying to a volcano called Nyamuragira in a region rife with dangerous militias.

After many active years, this volcano has stopped erupting.

The fire in its crater appears to be gone.

We need to collect some really critical data up there to understand what might happen in the future.

NARRATOR: Will Nyamuragira erupt again?

And if so, when?

♪ ♪ The people who live here are at risk.

Can scientists find a way to protect them before time runs out?

KAYLA IACOVINO: Right now, I just want to get things set up and going, so we can just get as much as possible in this really short time that we have here.

NARRATOR: Can they solve the mystery of "Volcano on the Brink"?

Right now, on "NOVA."

♪ ♪ Major funding for "NOVA" is provided by the following: NARRATOR: In a remote region of central Africa lies one of the most active yet least explored volcanoes on the planet: Nyamuragira, as spectacular as it is mysterious.

The volcano sits on the eastern border of the Democratic Republic of Congo, the DRC, near the shores of the vast Lake Kivu.

Fewer than 20 miles from the volcano is the town of Sake.

Its residents are all too familiar with the threat of its frequent eruptions.

Market traders Terese Kalume and Mama Noya have spent their entire lives here and witnessed the effects for themselves.

(speaking Congolese Swahili): NARRATOR: Nyamuragira eruptions blanket large areas of farmland in scalding-hot, choking ash, destroying crops, killing livestock, and bringing famine.

MAMA NOYA (speaking Congolese Swahili): NARRATOR: At the moment, there is just such a break.

It's been five years since the last major eruption.

Has the volcano gone extinct, or will it erupt again, more deadly than before?

♪ ♪ It's this question that an international group of scientists has come to investigate.

To do that, they have to be transported to the volcano by a United Nations peacekeeping team.

For British geologist Chris Jackson, it's a field trip like no other.

CHRIS JACKSON: It's actually in a fairly dangerous part of the the area, so the only way to get there is with a military helicopter.

My heart's racing just at the thought of getting on that helicopter.

NARRATOR: Few people have visited Nyamuragira-- for good reason.

This region of the DRC was the center of one of the bloodiest wars of modern times.

Fueled by a long colonial history, vast mineral wealth, political tensions, and international pressures, millions were killed and injured.

Although the war is officially over, widespread unrest continues.

Violent militia and rebel groups are operating in nearby forests, so the helicopter flies fast at treetop level to stay out of their gunsights.

Even at this speed, the helicopter provides a great vantage point to see one of the most volcanic places on earth.

American volcanologist Kayla Iacovino has studied volcanoes across the planet, but the volcanism here is on a scale she's never experienced.

IACOVINO: I thought I knew what this landscape was going to look like, but there's really way more volcanoes, way more melt here than I expected.

It's... the amount of magma production in this region is insane.

NARRATOR: As they reach Nyamuragira, they can see it's a giant shield volcano, so called because successive lava flows have spread widely across the landscape, giving its surface a sloping, shield-like shape.

The volcano rises to over 10,000 feet, surrounded by old lava flows that blanket almost 600 square miles.

At the summit, the main caldera created by previous eruptions is over a mile across, with 300-foot-high walls and, at its center, a 600-foot-deep crater.

♪ ♪ Usually such an active volcano near so many people would be covered in monitoring equipment, but because it's remote and dangerous, Nyamuragira has none.

This is a rare opportunity to gather data for predicting future eruptions.

We're hoping to land on top of the volcano, right next to its active crater.

We need to collect some really critical data up there to understand what might happen in the future.

NARRATOR: This is one of Africa's most active volcanoes, and it has a very complex history.

During the past 100 years, it's erupted from its flanks at least 30 times, flooding the surrounding farmland in molten rock, though this hasn't happened for the past five years.

In 2014, for the first time in almost a century, a small lava lake developed in the center of the crater.

So, the first thing the scientists want to check is whether that lava lake is still there.

♪ ♪ JACKSON: We're banking round now.

There's just sheer cliffs right down to the lava lake, which, from what I can see at the moment, seems to... seems to be crystallized, so seems to have turned into rock.

♪ ♪ NARRATOR: But does this have anything to do with the volcano's behavior?

With the lava lake apparently gone, will the volcano return to its cycle of huge, destructive eruptions?

Belgian geologist Benoît Smets, who works with local scientists, specializes in geo-hazards.

I think we should first follow the, the cracks...

Okay.

and then turn right right, try to avoid them.

Okay, cool.

NARRATOR: He discusses plans with Aldo Kane, a former Royal Marine, who's in charge of the expedition's safety.

Across the caldera, this is where the gas escapes, so it's quite dangerous.

Okay, all right, we'll get everyone out, and then we'll get a bit of a brief then.

Okay.

NARRATOR: It's not just the volcano that worries Aldo.

Just to be aware there are armed groups operating in and around the, the slopes of the volcano.

If you do see someone that's not from our group, then get on the radio, let me know.

There is a path and tracks going through here, so they are using it, it is accessible to them, so keep your eyes peeled.

NARRATOR: Any armed groups that saw them fly in could be heading to the summit, so they can't stay long.

So, we've got two hours, so we need to be back here, everyone at the chopper ready to go in two hours.

Cool?

SMETS: Okay, sounds good.

ALL: Thanks, Aldo.

NARRATOR: The urgent question for the team is whether the current break in large-scale eruptions is coming to an end.

♪ ♪ Kayla believes that the plume of gases released by the volcano will yield vital clues.

I want to find a place where I can actually get inside the plume and put the gas box, and that can tell me more about the different chemicals that are coming out of the plume.

Monitoring the changes in that gas chemistry is what tells us whether the system is changing, whether it's moving towards an eruption, whether there's new, new magma being input at the very base.

The gases really tell the whole story.

NARRATOR: Most volcanoes have a magma chamber, a reservoir of molten rock deep underground that fuels eruptions.

As the magma rises up towards the surface, it releases a mixture of gases.

A sudden increase in one gas, called sulfur dioxide, often signifies an imminent eruption.

Kayla wants to discover the concentration of sulfur dioxide in the gas plume, but it won't be easy.

I'm really... yeah, I feel, I feel like we're really pushed for time here.

There is a lot of gas coming out of here.

The problem for me is that once it gets to the top, it's, it's become pretty diffuse, which is why I'm having to chase the plume around.

NARRATOR: The team also needs to check on activity in the lava lake.

Kasereka Mahinda, a geophysicist, knows the summit better than anyone and can identify the best place to see into the crater.

MAHINDA: The best place to...

The best place is there, because one time I stayed there, you can see around the crater.

JACKSON: Okay.

Yeah.

NARRATOR: Most eruptions are driven by a build-up of pressure inside a volcano.

It's possible that the lava lake that appeared in 2014 acted like a safety valve, an open vent releasing pressure.

(volcano rumbling) When Kasereka was last here, the lava lake was still active, a small cone erupting in its center.

MAHINDA: I was here, you have active lava in the crater, very big, active lava.

NARRATOR: But now, from the helicopter, it looks as though the lava lake is no longer active.

If so, then the volcano may have lost its safety valve and could now be building towards a major eruption.

♪ ♪ With only two hours to try and find out what's happening, the team decides to split up.

Kayla heads off on her own to get a gas sample, while Kasereka leads the others to the vantage point overlooking the crater.

(indistinct talking) Kasereka works at the Goma Volcano Observatory, which carries out research on volcanoes across the region.

The observatory's job is not only to monitor the risk of eruptions but also the long-term effects of living in such a volcanically active area.

We look at the whole landscape, you know, it's about... they... all volcanoes in the landscape, that's one.

But mainly we focus on the two active volcanoes.

Still... we still also monitoring the rest.

We do also measure the quality of water people are drinking to advise the health authorities.

(children shouting playfully) NARRATOR: Mathieu Yalire studies volcanic gases.

Together with fellow volcanologist Dario Tedesco, he's investigating the volcano's effect on the water supply in Sake.

YALIRE: This is part of our job, to analyze water, all samples we can get around the region.

NARRATOR: Volcanoes tap the inner earth, releasing certain elements into the environment in much higher levels than normal.

Some can be harmful.

Mathieu and Dario are interested in one particular element: fluorine, that dissolves in water to form fluoride.

Many of the townspeople have brown, stained teeth.

Not a sign of neglect but possibly a condition known as fluorosis, too much fluoride in their diet.

♪ ♪ Around the world sodium fluoride is often added to drinking water.

At low concentrations, it helps prevent cavities, but at higher levels, it can cause problems.

Mathieu and Dario measure the levels of fluoride.

TEDESCO: The limit of this machine is ten part per million, and it says, "Over the limit."

NARRATOR: The water has levels of fluoride around ten times the recommended safe limits-- concentrations that can damage teeth, bones, joints, and even organ function.

And there is no easy remedy.

(children talking indistinctly) At present, we really don't have any solution, because the best solution would be to bring water from very far from here, in, uh... Masisi.

Masisi, Masisi area.

It's a very...

It's 20, 30 kilometers.

This is not only this village.

It's more or less 100,000 people or more that live in the area.

♪ ♪ NARRATOR: There's another effect of the elements thrown out by the volcano, one that benefits the community.

Between eruptions, the lava and ash break down, releasing nutrients into the soil and creating incredibly rich and productive farmland.

It's why the market is full of food.

For local people, this combination of risks and benefits can lead to a complex mindset of both fear and appreciation of the volcano.

(speaking Congolese Swahili): ♪ ♪ (car honking) NARRATOR: Back on Nyamuragira, the scientists are trying to figure out what the volcano will do next.

Kasereka Mahinda from the Goma Observatory is leading them to a viewpoint over the crater to help them assess if the lava lake that formed in 2014 is still active.

Oh, wow.

As you walk towards the edges of these craters, you get that feeling in your stomach, like you're about to go off the edge of the world.

♪ ♪ NARRATOR: The crater is dark.

It looks as though the activity Kasereka observed in the lava lake has now stopped.

To be sure, Benoît and his colleagues set up a thermal camera to check if there's any magma moving beneath the thin crust of black rock.

But that means getting uncomfortably close to the edge.

Just go careful on that edge, this entire edge, even under where your camera is there, is over-hanging.

That's millions of tonnes of rock there, and they're right on the edge of it.

NARRATOR: They check the temperature.

Intense heat would mean the vent is still active.

JACKSON: So what's the temperature down on the base?

SMETS: So everything has the quite same temperature, about 45 to 50 degrees Celsius.

NARRATOR: Although quite hot, 50 degrees Celsius, or 120 degrees Fahrenheit, is not hot enough for there to be magma near the surface.

It's totally dead.

No activity left at all.

And the question is now, is it just a break or is it just finished?

It means that the activity may change.

NARRATOR: With no active lava and a solidified crater floor, pressure could be building inside the volcano.

D'OREYE: The volcanic activity that we see is only a very very small part of the real volcanic activity.

There is much more happening below the surface.

NARRATOR: Some of the best clues of what's happening beneath the surface are the gases given off by the volcano.

They could reveal if new lava is rising up inside, threatening an eruption.

That makes Kayla's gas measurements all the more important.

But getting a sample is proving difficult.

IACOVINO: Unfortunately, the best place where the gas is coming up is also on, I think, the most precarious part of the entire crater rim.

It's why I'm not over there, where its gassiest.

Is that... these thick layers of ash, that looks like the most unstable, but I'm just trying to get as close as I can without being unsafe.

NARRATOR: Given the dangers of gathering data, Kasereka sets up a simple instrument he knows will be helpful in understanding the volcano.

I need to set up my... You're setting up here?

Yeah?

NARRATOR: The observatory is trying to establish a set of baseline measurements to spot changes in the volcano that could signal an eruption.

So what are you gonna set up here?

I set up the thermometer to measure the temperature.

So, temperature measurements?

Yes.

NARRATOR: Kasereka's focus is the fumaroles found across the crater, cracks and openings that can reach all the way down to the magma chamber.

Measuring their temperature can reveal what's happening deep inside the volcano.

MAHINDA: You know, we'll see if there, the temperature increase or not.

JACKSON: Okay.

So I can tell if the magma is coming up or not.

NARRATOR: As magma moves up inside the volcano before an eruption, the fumarole temperature increases dramatically.

But to spot that, a set of baseline measurements is essential.

We've lowered the probe in about a meter, meter and a half.

Yeah, this one will record every five minutes; Okay.

I try to reduce the minutes because we don't have enough time.

There's no screen on the data logger, so we can't see what the temperatures are at the moment.

We have to wait till we go back and plug it in.

Yeah, I have to go back to my office, Okay... okay.

'cause I have some software to analyze, yeah.

NARRATOR: This kind of data could ultimately help predict future eruptions.

♪ ♪ With little time remaining, Benoît is also determined to squeeze in one last experiment that could reveal pressure building within the volcano.

I would like you to follow the drone with binoculars to be sure I don't crash it.

♪ ♪ NARRATOR: Years of observations around the world have shown that before an eruption, increasing pressure can change the shape of a volcano, making it swell.

Perhaps the most extreme example is Mount St. Helen's in Washington state.

In 1980, the north side of the volcano bulged outwards some 270 feet just a month before its violent eruption.

Usually scientists are looking for much more subtle changes, which can take months of monitoring with a whole range of ground-based sensors.

(drone humming) That's not possible here, so Benoît has come up with a high-tech solution.

I'm using a drone to take pictures of the big crater in different viewpoints.

And with this set of images, I will be able to create a 3D model of the big crater.

NARRATOR: The computer model Benoît creates is a 3D snapshot of what the crater looks like today.

Comparing it to images captured on future visits will allow him to spot small changes in the terrain, which may precede an impending eruption.

♪ ♪ Benoît has just enough time to finish his survey.

The two hours are up, and weather is closing in.

KANE: We are a long way from the helicopter, and there's a huge bank of cloud (thunder rumbling) that's coming our way, so I think we take the weather window and we bug out.

NARRATOR: If the storm hits, the helicopter will not be able to take off, leaving the team stranded on the summit.

If you need me to carry anything, let me know.

Okay.

There's some space in my pack.

KANE: And we still need to pick Kayla up on the way as well, 'cause she's still over there doing her gas box.

NARRATOR: It's been a frustrating day for Kayla, who hasn't managed to capture any useful data.

IACOVINO (voiceover): It's just so hard to work in places like this, where access is nearly impossible, and then when you get access, you have two hours.

It's just not enough.

(helicopter engines humming) NARRATOR: Even so, this rare visit has been worthwhile.

They've confirmed that the lava lake that appeared in 2014 is no longer active, raising the possibility that the volcano may once again be building towards a major eruption.

If so, the new monitoring equipment they've left behind may help forecast that event.

♪ ♪ As the team departs, they turn their attention to another goal of their expedition: investigating what makes this part of Africa... so intensely volcanic.

JACKSON: It's amazing from up here, the view you get.

From the ground, it was spectacular, but from here, it is absolutely something else.

This is flat land with these volcanoes just punching through absolutely everywhere.

NARRATOR: The entire area is filled with evidence of volcanic activity.

Each one of these small hills is an extinct volcanic cone.

But what is the impact of living in such a volcanic landscape?

♪ ♪ (blowing whistle) ♪ ♪ To find out, the team travels to Goma, a city of almost a million people that lies south of Nyamuragira and another giant volcano, Nyiragongo.

Nyiragongo last erupted in 2002, sending rivers of lava into the heart of Goma, causing death and destruction.

(people shouting) The city has since recovered and is growing.

♪ ♪ Today Chris and Aldo are heading to a local boxing gym.

Wow, check this out.

NARRATOR: They're here to learn about the relationship between the area's volcanic history, its ongoing instability, and what local people are doing to address the trauma of violence.

♪ ♪ Continuing unrest here means that children often have little choice but to fight in the armed gangs and militias that operate around Goma.

(man calling practice drills) Some of the men in this gym were once child soldiers.

The discipline of boxing provides a way for them to escape their violent past.

The continuing unrest and use of child soldiers are driven by the extraordinary mineral wealth of the region, especially one mineral that plays a crucial role in modern life.

(people talking indistinctly) JACKSON: This is the mineral coltan, this dark-colored mineral here.

It's the kind of mineral I'd expect to find in an area like this.

And any touchscreen phone, laptop, anything with a transistor will have tantalum, which is the tan of the coltan.

(man calling drills) ♪ ♪ NARRATOR: The explosion in smartphones and other electronics means coltan is in constant demand.

Even small mines can generate huge profits, but in this region, militias control many of the mines.

The income means they can recruit young people, fueling the ongoing violence.

JACKSON: It's very hard to hold this in your hand, knowing how much we desire it, and knowing what it leaves behind and where it comes from.

(chattering) Bonjour, Kibo.

(both speaking French) NARRATOR: The head of the gym, Kibomango, is a former child soldier who now tries to help young people rebuild their lives.

KANE: And you're happy for us to join in some, some training?

Joining, joining, no problem, no problem, yes.

NARRATOR: Aldo's fascinated by Kibomango's story, because like him, Aldo became a soldier at an early age.

KANE: So, Kibo, when did you join the army?

(speaking Congolese French): Which is two years younger than I was when I joined up.

How long did you serve in the, uh, in the army?

(Kibomango calling drills) NARRATOR: Although Aldo joined the Royal Marines at 16, legally he couldn't see frontline action until he was 18, unlike Kibomango and the young people here.

(calling drills) KANE: I wouldn't mind betting that what Kibomango's doing here to run this boxing club is partly for his own therapy.

As a child soldier, he's going to have seen some pretty nasty stuff.

(Kibomango calling drills) NARRATOR: After the workout, Aldo has a chance to take on the man himself.

Kibomango is a former Congolese champion.

KIBOMANGO (speaking Congolese French): ♪ ♪ (applause) NARRATOR: In a poor and job-starved area, militias are one of the few sources of money and employment.

It's a vicious cycle.

The chaos in the region helps gangs control the mineral wealth, and in turn the mineral wealth allows the gangs to maintain the chaos.

There are efforts underway to break the cycle by promoting a more sustainable use of the resources created by the volcanoes.

They focus on the Virunga National Park, a vast nature reserve covering around 3,000 square miles.

Its fertile volcanic soils make it one of the most diverse parks on the planet and one of the few places where it's still possible to see mountain gorillas.

♪ ♪ These highly social animals thrive in this lush forest habitat, eating mainly leaves, shoots, and stems.

Thousands of people come to see the gorillas, generating cash that's used to create local jobs.

But following attacks by poachers and criminal gangs, the park had to close temporarily, another setback for a region where volcanic riches have the potential to create a very different future.

♪ ♪ But what makes this area so volcanically active in the first place?

It's a question the scientists are determined to answer.

From the helicopter, I got a, like, almost a once-in-a-lifetime view of the entire volcanic landscape.

And what really struck me was the amount of volcanism and the amount of volcanoes here.

We have hundreds and hundreds of them.

♪ ♪ NARRATOR: This extraordinary concentration of volcanic activity is related to a vast chain of mountains and valleys.

The East African Rift.

It's a massive geological feature... stretching nearly 4,000 miles up the eastern side of the continent, and the Goma volcanoes are at the midpoint of the rift.

The earth's outer crust is divided into giant slabs called tectonic plates.

Along the East African Rift, the tectonic plate is splitting, and the two sides of the rift are moving away from each other as the continent slowly rips apart.

If the split continues over millions of years, a new ocean and continent will form.

But many rifts don't fully develop.

The waters of the Mississippi now fill the scar left by a failed rift when a huge section of what's now the Midwest started to tear apart around a billion years ago-- and then stopped.

Will the East African Rift also fail?

The answer hinges on a controversial theory.

JACKSON: So, one thing it could be is something called a mantle plume, that's a column of deep, hot material, which is rising up towards the earth's surface.

As it comes to the earth's surface, it can weaken the plate, and it can actually force the plates apart, but some of that melts, some of that magma can also come out onto the earth's surface and build volcanoes.

NARRATOR: The earth's mantle lies between its dense core and its thin crust.

The presence of a mantle plume, a huge column of heat and melted rock rising from deep beneath the region, would explain why there are so many volcanoes here.

♪ ♪ If a plume exists, then the enormous heat and energy it brings could keep the rift active long enough to split Africa apart.

JACKSON: Like all geological theories, we really need to go out into the landscape and look for additional evidence.

♪ ♪ NARRATOR: So along with Kayla, Chris goes on the hunt for evidence of a distinctively deep upwelling of heat and magma, the signature of a mantle plume.

The first place they want to look is one of the extinct volcanic cones, common in the region.

IACOVINO: I'm really interested to see some of the smaller cones, because we've been looking at Nyiragongo, Nyamuragira, you know, the big boys.

There's so much information that the smaller cones can have too.

NARRATOR: Their destination is Lac Vert, the "Green Lake," just outside Goma.

Wow, look at that.

♪ ♪ NARRATOR: Today it's a 300-foot climb down from the crater rim to the lake.

People bathe and wash clothes here, but around 500 years ago, this was an active volcano.

(laughter) Chris and Kayla want to know what sort of eruption formed Lac Vert, and whether it could be linked to the presence of a mantle plume.

They also want to find out if this volcano could erupt again, and what that would mean for the city of Goma.

They start by examining the rocks that make up the volcanic cone.

For many years the sides have been quarried, so with a little effort... You got to go over the head.

Over the head?

Yeah.

(spitting out dust) NARRATOR: ...it's easy to get their hands on some samples.

None of this is actually lava, this is all ash, these are all ash deposits.

And that means this was magma that came up through the ground, was exploded onto the surface and absolutely ripped apart into very, very fine pieces that you can see here.

Imagine how much energy it would take to take solid rock, and, and just explode it into these tiny, tiny bits.

That screams to me that there was an interaction with the water when this actually erupted.

NARRATOR: Goma sits in the middle of a string of craters like Lac Vert, each formed by explosive eruptions, and each capable of delivering a devastating blast.

The eruptions are caused by magma welling up from underground.

When the magma hits a layer saturated with water, the extreme heat instantly turns the water to steam, triggering a powerful explosion.

♪ ♪ This generates huge amounts of fine-grained ash, just like the deposits found here-- clear evidence that Lac Vert was formed by this type of explosive eruption.

The sides of the crater also reveal something else: this wasn't a single event.

JACKSON: What strikes me, as somebody who's interested in sedimentology, is how many layers of rock there are like this.

The layering in the rock behind us, I wouldn't want to go over there and count them, but there's clearly hundreds and hundreds of giant explosions associated with this catastrophic eruption.

NARRATOR: It may be about five centuries since the last eruption, but the danger of a new one is very real.

We've still got lots of volcanism here, we're still right next to the lake, we can see in Lac Vert, the water table is right here.

There's all the ingredients there for this to happen again.

♪ ♪ NARRATOR: A powerful new eruption along this densely populated shoreline, or in the city of Goma itself, would have a devastating impact.

But there might be warning signs.

As magma moves up towards the surface through underground cracks, it forces the earth apart, creating swarms of mini-earthquakes, which instruments called seismometers can detect.

♪ ♪ Joshua Subira of the Goma Observatory explains that seismometers have been placed next to the volcanoes and along Lake Kivu near the city of Goma.

SUBIRA (speaking French): NARRATOR: This network of seismometers is an essential tool in the effort to forecast eruptions here, where so many lives are at risk.

But neither seismometers nor cones like Lac Vert can tell Chris and Kayla whether the heat and magma that create these volcanoes is coming from a mantle plume.

♪ ♪ IACOVINO: I think we need to get deeper into this landscape and try and uncover what's really driving everything from the biggest, biggest scale sense.

NARRATOR: One place they have yet to explore could hold the answer.

Lake Kivu.

Joining Dario Tedesco and Mathieu Yalire from the Goma Volcano Observatory, Chris and Kayla head out on the waters of the vast lake that lies directly south of Goma.

Lurking in its depths is a huge hidden danger, worse than anything they've seen so far.

It could also hold the vital clue about whether a mantle plume is driving the volcanic activity here.

IACOVINO: Maybe now we're ready to put this thing in the water, yeah?

NARRATOR: The key is to get a water sample from the deepest part of the lake.

Let's try not to lose everything.

♪ ♪ NARRATOR: Mathieu and Dario drop an open sample bottle down to a depth of 55 meters, about 180 feet.

50 meters... and here we have 55 meters.

If you want to see it...

I'll stand it.

There it goes.

Yep.

We'll leave it.

NARRATOR: A heavy weight is then sent down the rope to trigger the mechanism that closes the bottle.

IACOVINO: You can feel the weight of that, you can feel as I'm dragging it up, pushing the water out of the way as I bring this thing up.

I think at five.

Yes, here is it.

And look... wait, wait wait.

Look at the... the gas coming out.

Come, come here.

Oh, my gosh.

(burbling) Yes, you see?

NARRATOR: Like a carbonated drink, the bubbles consist of gas suddenly released from the water sample.

This is the danger lurking in the lake.

TEDESCO: When you open... (gas hissing) IACOVINO: You can hear it.

JACKSON: It's like opening a bottle of pop.

Yeah, exactly.

You see see the...

It looks like a soda, it's all completely carbonated.

TEDESCO: It's 99% is carbon dioxide.

99%?

Yeah.

NARRATOR: The bottom of the lake contains the potentially lethal suffocating gas, carbon dioxide.

It's given off by the constant volcanic activity beneath the Goma region, seeping into the lake from the underlying bedrock.

Its presence is a serious threat.

♪ ♪ Lake Nyos in Cameroon also has volcanic carbon dioxide trapped in its depths.

On the night of August 21, 1986, a giant lethal pulse of the gas escaped from the depths of Lake Nyos, flowing over the shoreline and into surrounding fields and villages.

Almost 2,000 people suffocated as they slept, and Lake Kivu is far bigger than Lake Nyos.

All the lake has a huge amount of CO2, about, uh... 256 cubic kilometers.

♪ ♪ NARRATOR: That's enough gas to cover an area more than 50 times the size of New York City in a suffocating layer 20 feet deep.

The risk is that a volcanic eruption or earthquake could destabilize Lake Kivu, releasing its carbon dioxide gas, threatening the lives of thousands of people.

♪ ♪ But there are efforts underway to prevent that, and the group is going to see an experimental project.

JACKSON: This is one of the most amazing things I have ever seen.

We're in this glass flat bay, and all of a sudden, you come to this platform, and it's just... all hell has broken loose.

IACOVINO: It's a massive amount of pressure that... it's almost like a geyser just spewing stuff.

TEDESCO: It is a geyser.

It is a geyser?

It is a geyser, and we don't see the geyser itself, just because there is something on top.

The core that is just 50 meters in order not to let the geyser blow.

♪ ♪ NARRATOR: Salty water, another product of volcanic activity, seeps into the deeper sections of the lake, forming a dense layer that acts like a lid, trapping the carbon dioxide below.

The aim is to find a way of slowly releasing the trapped gas at concentrations that don't endanger people.

A pipe runs down from the platform into the carbon dioxide-rich layers.

The water saturated with gas flows up the pipe in a controlled release, allowing the carbon dioxide to mix with the air, making it harmless.

YALIRE: This is a pilot to see if there is a way to de-gas this part of the lake.

If it succeeds, we make a very big one to put the... a very big amount of CO2 in the air.

NARRATOR: The controlled release of the carbon dioxide reservoir at the bottom of the lake could prevent a future disaster; but it's a massive project and very expensive, although efforts to commercialize methane, or natural gas, also found at the bottom of the lake, could help defray the cost.

Meanwhile, the dangerous carbon dioxide in Lake Kivu provides a valuable clue to what's driving the area's many volcanoes.

IACOVINO: Where is the carbon dioxide actually coming from?

Most of the carbon dioxide that we collect all over the lake comes straight from the mountain.

This is probably the best findings we have.

NARRATOR: The carbon dioxide they collect here bears a distinctive chemical signature, revealing its origins were deep in the mantle, the thick layer between the earth's crust and its core.

Here in the Goma region, the earth's mantle is at least 20 miles beneath the surface.

Only a giant mantle plume, a huge upwelling of magma, could bring gas up from such a depth.

JACKSON: The idea that there's a mantle plume underneath here, there's some evidence at the bottom of the lake, I just never thought that would be something we'd find out.

It's really the last, the last piece of the puzzle in this landscape, I think.

NARRATOR: The existence of a mantle plume bringing up extreme heat from the depths means the East African Rift will likely continue.

Africa may eventually tear apart.

Fortunately, not for many millions of years.

But the scientists believe the energy unleashed by the plume explains the extraordinary volcanic activity witnessed during their expedition.

IACOVINO: The CO2 that's sitting in the bottom of this lake, all of the gases, the sulfur that you can smell, it's all coming straight out of the mantle.

It's just streaming off, this upwelling plume of material that's delivering heat, energy, chemistry to the whole landscape.

To the volcanoes we've been on, the bottom of this lake, it's all connected to the same huge process.

♪ ♪ ♪ ♪ NARRATOR: After three weeks the expedition has come to an end.

(sighs) It's been grueling, but the team has gained unprecedented insights into the volcanoes that dominate this region.

On Nyamuragira, their aerial survey is an important step toward predicting future eruptions.

And in the depths of Lake Kivu, they found evidence that a vast mantle plume drives the region's intense volcanism, which may ultimately split Africa in two.

(laughing, chattering) The people here live side by side with geological forces that are reshaping the face of our planet.

If you live in this amazing, amazing place, you can live with the hazards, but the hazards cannot be at the front of your mind all the time.

It would just stop you from doing anything, I'd guess.

NARRATOR: The knowledge gained on the expedition, working with local scientists, is vital to better prepare the people of Goma for future eruptions... which are bound to occur.

MAHINDA: We have two active volcanoes now.

The city became very big, many people in Goma.

The gas in Lake Kivu, it will make problem.

Maybe we have to monitor and continue to monitor, to make warning in time.

NARRATOR: These forces can be understood but never completely controlled.

This will always be one of the world's most extraordinary places to live... a landscape of both deadly hazards and incredible wonders.

We have this absolutely vast volcanic landscape here, and the people living right on top of it.

It's my hope that people can learn to understand that landscape better, in order to not only protect themselves but also to reap the benefits from it.

Hey!

♪ ♪ NARRATOR: From pain pills to fentanyl.

MAN: A parent's worst nightmare, to find my kid not breathing.

NARRATOR: A deadly epidemic.

MAN: Overdose is the number-one cause of death for people under 50.

NARRATOR: But there are solutions.

WOMAN: We have life-saving medications.

NARRATOR: A brain disorder that can be treated.

WOMAN: Let's embrace addiction as a disease, bring it within the house of medicine.

NARRATOR: "Addiction," next time, on "NOVA."

This "NOVA" program is available on DVD.

To order, visit shopPBS.org or call 1-800-PLAY-PBS.

"NOVA" is also available on Amazon Prime Video.

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