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

♪ ♪ 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, but no one knows the answer, because these are among the least studied volcanoes in the world.

(outboard motor rumbling) Now an international team of scientists is investigating these giants.

Their goal?

To predict future eruptions and save lives.

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

♪ ♪ NARRATOR: They're heading to the volcano known as Nyiragongo, which towers over a rapidly growing city of a million people.

An eruption in 2002 caused death and destruction.

CALEB KABANDA: I could see the lava flowing and wrecking all houses.

You could hear the noise.

NARRATOR: To penetrate the volcano's secrets, the scientific team descends into the crater itself.

It's treacherous.

CHRIS JACKSON: Rock!

Everything's moving.

Nothing is stable.

NARRATOR: But with lives in the balance... JACKSON: Rock!

NARRATOR: ...the stakes could not be higher.

Oh, man, look at these rocks.

Just precariously balanced.

♪ ♪ NARRATOR: "Volcano on Fire," right now on "NOVA."

♪ ♪ Major funding for "NOVA" is provided by the following: ♪ ♪ NARRATOR: Deep in central Africa is one of the most spectacular, active, and dangerous volcanoes on Earth.

♪ ♪ KAYLA IACOVINO: That's terrifying.

There's literally nothing like this in the world.

NARRATOR: This volcano, named Nyiragongo, threatens almost a million people in this region.

♪ ♪ Twice in recent memory it has devastated the city of Goma.

(roaring) REPORTER: The red river keeps flowing, pouring out of the volcano and down towards Goma.

(clamoring) NARRATOR: Violent conflict in this region has often made it too dangerous to mount any large scientific expeditions.

But a temporary peace opens the door for a groundbreaking investigation.

♪ ♪ Because this giant is one of the least understood volcanoes in the world, both threatening and life-giving.

WOMAN (speaking Congolese Swahili): NARRATOR: A team of scientists will spend a week on Nyiragongo, the volcano that towers over Goma.

Their goal is to find ways to predict when this volcano will next erupt.

IACOVINO: A huge spike in the amount of sulfur dioxide, that could be something that happens before an eruption.

NARRATOR: Their search for answers will take them deep inside the crater... JACKSON: I've just come over the edge.

NARRATOR: ...and into great danger.

ALDO KANE: If any of this rock goes here, that's it for both of us.

(exhales) (sighs) ♪ ♪ ♪ ♪ NARRATOR: Now the team is on its way to the 11,400-foot-high Nyiragongo.

It lies on the Democratic Republic of Congo's eastern border with Rwanda.

Its steep cone, created by successive eruptions over centuries, rises over a mile above Goma and the surrounding landscape.

♪ ♪ Forecasting eruptions is difficult and uncertain even for well-studied volcanoes, but it can save lives by giving people time to get out of harm's way.

23, 24, 25... NARRATOR: To achieve that, the scientists have brought an array of equipment to investigate this volcano in many different ways.

(laughing) Just take care that the men take the heavy bags...

Yes.

NARRATOR: Leading the science team is Belgian volcanologist Benoît Smets.

SMETS: Studying Nyiragongo is very important for me, because this volcano is very dangerous.

It, uh, threaten a lot of people.

Everything we do to understand this volcano is very important to avoid another disaster like in 2002.

NARRATOR: Joining him are scientists from the local volcano observatory, who want to install instruments on the crater rim.

Joshua Subira has studied this volcano for several years and knows firsthand how difficult the climb will be.

SMETS (speaking French): (speaking French): SMETS: SUBIRA: SMETS: Pourquoi?

SUBIRA: (talking in background) Actually that's the one I'm worried about, because I don't want someone to drop it.

NARRATOR: American geologists Jeff Johnson and Kayla Iacovino are worried about damage to the instruments they brought to help identify warnings of an eruption.

(man speaking indistinctly on radio) No, the sun's out down here.

Did you have a good night last night?

MAN (on radio): Absolutely hammered it down.

NARRATOR: Former Royal Marine Aldo Kane is in charge of getting everyone and everything to the top of the volcano.

KANE: We've got science kit, expedition kit, rigging kit, food, water, nearly four tons of kit that's going up the hill today.

NARRATOR: Nyiragongo's crater rim is 6,500 feet above the jungle and only accessible on foot.

(talking in background) ♪ ♪ It takes six hours for the team to reach its steep upper slopes.

(panting) The weather's closed in.

It's cold.

But their reward is a view of one of Earth's great natural wonders.

JOHNSON: Oh, my God, that is so incredible.

IACOVINO: There's literally nothing like this in the world.

♪ ♪ JACKSON: There are six permanent lava lakes on Earth.

You are standing looking into one of those six.

Of those six lava lakes, they are all babies compared to this size of... Yeah, they are.

JOHNSON: All other global lava lakes could fit into this lava lake, with tons of room left over.

This lava lake is enormous.

IACOVINO: The sheer size of it, I think, is just hard to fathom.

♪ ♪ NARRATOR: The crater is almost a mile in diameter.

♪ ♪ Inside is a permanent cauldron of molten rock.

And it constantly churns at almost 2,000 degrees Fahrenheit.

JACKSON: It's ferocious-- it feels very alive.

I mean, even when we're silent, there's that constant roar.

It's just, doesn't let up at all.

♪ ♪ (bubbling) NARRATOR: British geologist Chris Jackson has studied volcanic landscapes across the world.

♪ ♪ For him, volcano prediction is a scientific and humanitarian challenge.

Yesterday, he traveled in through the city that lies at the foot of the volcano.

JACKSON: It's amazing to think that there's such a threat posed by that volcano, yet because of that volcano, you can build houses and some sort of infrastructure.

♪ ♪ NARRATOR: Goma, a city of almost a million people, is growing and modernizing rapidly.

Houses are going up everywhere, with foundations made from volcanic lava.

JACKSON: This is incredibly exciting to be joining an expedition like this, to do cutting-edge, critical science for the people living in the shadow of this giant volcano.

NARRATOR: In the outskirts of the city, the volcano is virtually a next-door neighbor.

JACKSON: This place may look like a building site, but it's a building site sitting directly on top of this, this lava, which was erupted only 15 years ago from the volcano of Nyiragongo.

For the people who live in the city of Goma, this is just a disaster waiting to happen.

NARRATOR: A disaster that has happened many times before.

♪ ♪ REPORTER: Daylight, and some of the first pictures reveal a black blanket of lava covering entire neighborhoods.

(siren blaring) (people screaming) NARRATOR: When Nyiragongo erupted in 2002, lava flows split the city in three, destroying many homes and causing about a hundred deaths as people fled for their lives.

REPORTER: The red river keeps flowing, pouring out of the volcano and down towards Goma.

♪ ♪ (blowing whistle) KABANDA: I could see people walking on this main road fleeing the volcano.

NARRATOR: Caleb Kabanda, a local journalist, witnessed the whole eruption from the center of Goma.

KABANDA: You know, it was a lake of lava destroying everything.

I could see lot of smoke, and the lava flowing and wrecking all houses.

You could hear the noise.

I was very scared.

REPORTER: The lava has been flowing for two days now.

It shows no sign of stopping.

(fireball whooshes) KABANDA: Some places you could see the fire burning.

Many people panicked, because it was a disaster to the whole city.

You see all the houses you knew were destroyed.

It was really, uh, terrible and I thought that this is the end of Goma.

REPORTER: As if the people of Goma had not suffered enough, this was a day that brought them more death, more tragedy.

Fireballs filled the sky after a petrol station here exploded.

Fuel cans leapt into the air.

NARRATOR: Eruptions like this are no surprise, because Nyiragongo is part of one of the largest volcanically active zones on Earth, the East African Rift, a vast scar created by the pulling apart of the land.

It's believed this is happening because deep in the Earth, volcanic magma is rising, pushing up on a massive continental plate, slowly splitting it in two.

If it continues, Africa will eventually break apart, creating a new ocean, the first to form in over 30 million years.

Nyiragongo is in the center of this giant geological tear.

There have been two major eruptions in the last 40 years.

There is little doubt it will erupt again with the same catastrophic consequences.

♪ ♪ Back on the volcano, overlooking the crater, the science team considers tomorrow's descent.

It will be a difficult and hazardous climb, but it's the only way they can study the lava lake up close.

JACKSON: That is full on.

KANE: From where we are here to get down there, it's over 400 meters, so that's what, four times Big Ben?

And that's where we hope to camp, down there on that, that second level.

IACOVINO: That's terrifying, that is absolutely terrifying.

♪ ♪ (churning) NARRATOR: As terrifying as it appears, the volcano is also mysterious.

As Chris explains, it's not completely understood why it erupts at all.

JACKSON: This, this is a volcano.

Beneath this volcano, and all of the volcanoes in the world, is a magma chamber, or some molten body of rock.

It's magma from this chamber which rises up into the volcano.

and it's changes in pressure within this magma chamber that gives rise to eruptions out of volcanoes.

Here at Nyiragongo, we have a lava lake.

In theory, there shouldn't be any pressure building up in here.

All of that pressure should be just oozing and venting out of the surface without eruptions.

However, we do know that this volcano, it's common to have dangerous eruptions from the flank of the volcano.

♪ ♪ NARRATOR: Flank eruptions, like the one in 2002, can be lethal.

The science team believes that intense activity deep underground increases pressure so rapidly that lava forces its way out of the volcano's side.

So they need to find a way to measure changes in pressure deep underground.

But there's a problem.

It's very difficult, or almost impossible, to directly measure changes in magma pressure directly within the chamber.

One thing we can do is to look at changes in the lava lake itself.

NARRATOR: The constant activity of the lava lake and the gases venting from its surface are not only a spectacle, they provide clues about pressure changes inside the magma chamber, partly due to the build-up of gases such as carbon dioxide, water vapor, and sulfur dioxide.

And some of these volcanic gases pose a deadly threat.

♪ ♪ In fact, these gases are found in the ground throughout this whole region, and they seep out, creating a potentially fatal danger for the people and animals that live here.

Volcanologist Dario Tedesco and Mathieu Yalire are in a village just outside Goma to warn the local people about this deadly side effect of living here.

Mathieu has gathered them around a pit that appears innocuous, but contains an invisible and deadly gas.

♪ ♪ YALIRE (speaking Congolese French): ♪ ♪ NARRATOR: The gas is carbon dioxide, though the locals have another name for it-- mazuku.

Mazuku mean "evil wind."

NARRATOR: The gas slowly seeps up through the cracks in the rocks, from the pockets of magma that underlie this whole area.

At concentrations of just seven percent, it silently kills within minutes by suffocating its victims.

♪ ♪ Mathieu and Dario head down into the pit.

(equipment beeping) TEDESCO: 20% already.

50%.

NARRATOR: And discover levels of carbon dioxide... 70%... NARRATOR: ...as high as 90%.

(Tedesco speaking in background) (equipment beeping rapidly) NARRATOR: More people are killed by mazuku than by volcanic eruptions.

And because carbon dioxide is heavier than air, it pools in depressions all around this area, so children are especially vulnerable.

YALIRE (speaking Congolese French): ♪ ♪ NARRATOR: Here at the Goma Volcano Observatory, scientists have set up a network of seismic stations to keep an eye on activity across the region.

These stations detect tiny tremors set off by lava as it forces its way towards the surface, a telltale sign of a new eruption.

Geologist Joshua Subira monitors it all.

(speaking French): To locate.

(speaking French): ♪ ♪ NARRATOR: On top of Nyiragongo, Joshua and the science team are planning to install the first seismic station on the volcano itself.

But getting to the right location means a difficult and dangerous hike along the razor-edged crater rim.

NICOLAS D'OREYE: If you're a goat, it's easy-- just watch your feet, keep always at least one hand free to recover your balance.

NARRATOR: After a precarious two-hour walk, Belgian scientist Nicolas d'Oreye finally finds a good spot.

♪ ♪ D'OREYE: This is simply a piece of flat stone, so we have a flat surface to have the seismometer resting on.

NARRATOR: The network needs sensors in many different places so the scientists can establish the depth and location of volcanic activity.

D'OREYE: We are interested in knowing exactly where the signal comes from.

If you have several fractures making progress toward the surface, it might be a problem if it start to become shallower, and then that's an eruption.

NARRATOR: For Joshua, this network of seismic stations is vital, because more stations give scientists a better chance of predicting an eruption.

SUBIRA (speaking French): NARRATOR: Although the seismic network in the region is improving, it isn't a foolproof way of detecting eruptions.

If the lava flows through existing cracks in the rocks, the seismometers won't register any tremors.

So the team needs additional ways to monitor the volcano, to evacuate people and save lives in the city of Goma.

(children exclaiming) ♪ ♪ First light on the crater rim.

The priority is to get down to the lava lake as soon as possible.

♪ ♪ The weather conditions are miserable, but at this altitude, it can get a lot worse.

KANE: The temperature is slightly warmer in there, but it can change like that, and it can go down to freezing.

My team were down there yesterday and were caught out in a hailstorm.

(hail pelting) Again, just, stuck in with the heaviest hail.

(hail pelting, Kane groans) NARRATOR: Torrents of water stream down the loose crater wall, creating treacherous rockfalls.

KANE: There are waterfalls coming all down the side of the volcano, knocking massive rocks, coming flying towards us.

NARRATOR: For Aldo, the risk of bad weather means a change of plan.

KANE: What we want to do is cut the amount of people that are going down into the volcano to essentials only, and that-- Kayla, I was speaking to you earlier on, and you mentioned you can do a lot of your stuff up on the top here.

So, um, I'm happy to, to keep you up here and not take you down there, because of that.

IACOVINO: I can do all of the work that I need to do basically on the rim.

♪ ♪ There's definitely some mixed emotions behind me not being able to go down.

There's a bit of relief, because it, it is so dangerous to do.

Um... And, and there's also a bit of disappointment, because, you know, what an amazing experience to spend a couple of nights in the crater next to a lava lake, it's something I could never have dreamed of being able to ever do.

♪ ♪ NARRATOR: When the weather clears a bit, Aldo decides to start the descent.

(equipment clanking and zipping) JACKSON: I can feel my heart rate going up just putting the harness on.

Wait till you look over the edge.

Yeah.

KANE: Last bit before we go down, kicking rocks off...

If you do kick a rock off, big shout, "Rock!"

Okay.

If one of these rocks hits someone on the head, it will kill them, even with these helmets on.

Okay.

I think I'm ready.

(chuckles) You should enjoy the view first before we go over, because after that you're gonna be fairly dizzy.

Okay.

♪ ♪ NARRATOR: Nyiragongo's crater has three levels.

Tier one is a small outcrop almost a thousand feet below the crater rim.

A sheer, 250-foot drop below tier one is tier two, where the team will be camping.

And finally, tier three, the bottom level that surrounds the lava lake.

KANE: How you feeling?

A blend of excitement and nerves, I'll be honest with you, yeah.

♪ ♪ NARRATOR: Climbing down is the most dangerous part of the expedition.

(sighs) NARRATOR: But even more so for Chris... KANE: Nice and gently, Chris.

NARRATOR: ...who's not an experienced climber.

JACKSON: Whew!

(gasps) KANE: How does it feel?

JACKSON: Better now-- I've just come over the edge.

NARRATOR: And in such a remote location, if something goes wrong, there are no rescue teams, no helicopters, to take an injured climber out.

JACKSON: Sorry.

Try not to do that, because there are sections if you do that, that the whole slope will go.

Yep.

KANE: All of this is just waiting to fall.

(wind blowing, ropes rubbing) (wind gusts) JACKSON: Whoa.

That wind's just picked up.

JACKSON: Yeah, yeah, I can feel it.

Be careful not to kick anything.

If you kick anything, it's coming down on my head.

Yeah, okay.

Everything's moving.

JACKSON: Nothing is stable.

Rock!

(rocks tumbling) KANE: Oh, Jesus.

JACKSON: Sorry.

KANE: Have a look back up.

JACKSON: Oh, yeah, yeah.

(Jackson panting) ♪ ♪ KANE: Yeah, that's good, that's good.

(Jackson lands hard): Ow!

You okay?

Yeah.

♪ ♪ NARRATOR: With the weather holding, the rest of the team also starts the descent.

KANE: Just watch your feet coming down.

(rocks tumbling) (blows) NARRATOR: After almost three hours on the ropes, Chris is more than halfway down... Something to tell the grandkids.

NARRATOR: ...and about to be lowered down about 250 feet to the campsite below.

KANE: That's where we're heading to, to the base camp down there.

Okay.

Okay.

First it's going to be Chris, over.

NARRATOR: To do this safely, Chris is suspended on what's known as a Larkin frame... Make yourself comfy.

Don't worry.

NARRATOR: ...a piece of gear brought down by Aldo and his climbing partner Daz during preparation for the descent.

♪ ♪ DARREN PHILLIPS: Clear now!

Thank you.

Rock!

(rope sliding rapidly) Oh, my gosh.

KANE: Steady.

Okay.

Chris is down.

Thanks, Daz.

That is spot on, that is very good.

Oh, too intense!

Don't look back.

Don't look back.

Don't look back.

♪ ♪ JACKSON: So, I made it down to T2.

Daz is just, uh, sorting out the ropes to be sent back up for the next victim.

But the first thing I noticed, as soon as I come down to this level, these giant chasms, maybe a meter, maybe two meters wide.

You can see in the distance there, the campsite that's been set up.

NARRATOR: Tents have been pitched between two potentially fatal hazards: the vertical drop down to the lava lake is less than a hundred feet away; and just behind the tents lies a field of fallen rocks from the crater wall.

Despite the danger, for Chris, the landscape is literally out of this world.

JACKSON: If I was to compare this environment to anywhere else, I'd say Mars.

There's just blacks and whites and reds, you know, it's very simple colors everywhere.

♪ ♪ There is no vegetation whatsoever.

There doesn't seem to be anything living down here.

One of the reasons is actually what I'm smelling.

It's, it's sulfur.

And all around us, there's these vents which are spewing out sulfur into the air.

NARRATOR: Sulfur gases, which Chris smells, are just some of the toxic gases released from the volcano.

By now, Benoît has also descended down to tier two, hurrying to prepare his first experiment, because time in the crater is limited.

He hopes to monitor the level of the lava lake to learn what's going on inside Nyiragongo.

Unlike most volcanoes, Nyiragongo erupts from cracks in its flanks.

But like all volcanoes, it erupts when pressure builds in its magma chamber deep underground.

♪ ♪ If the team could measure that pressure, it would provide a warning sign of an eruption, but the magma chamber is at least a mile below the surface, so it's impossible to measure any pressure changes directly.

Benoît suspects, though, the lava lake itself may reveal those pressure changes.

SMETS: Here we have the chance to have this big lava lake, and you can see the lava lake as a magmatic chamber at ground surface.

NARRATOR: The plan is to use time-lapse photography to record any changes in the lake level.

But when the lava lake is wreathed in venting gases, it's hard to see.

To penetrate these clouds of gas, Benoît's cameras detect light in the infrared part of the spectrum-- invisible to our eyes, but not to the camera.

♪ ♪ SMETS: I made these boxes myself.

My box is made of a microcomputer, that will control everything; a real-time clock to have an accurate time; and a camera, it's a small camera like you have on your smartphone.

And it will take every ten seconds, and by comparing these pictures, I will be able to see the variations of the lava lake level.

KANE: Okay, Daz, lowering Jeff out.

Okay, come up to it.

(Phillips talking on radio) NARRATOR: As evening falls, the other team members finally descend.

KANE: Lowering you into the cauldron.

Keep your feet on the rock.

NARRATOR: Aldo is the last to come down, in complete darkness.

(rope rubbing) The next morning, Benoît returns to his cameras to see if they've worked, and whether they can reveal anything about pressure in the magma chamber.

Whoa.

We've got a beautiful lava lake level drop compared to yesterday at the same time.

It's great.

So we recorded something special.

♪ ♪ NARRATOR: To Benoît's expert eye, the camera has recorded a drop of about 15 feet within the last 24 hours.

♪ ♪ Benoît believes the most likely explanation is that there has been a slight drop in pressure within the magma chamber.

A slow rise and fall in the level of the lava lake suggests stability, as opposed to what's known as gas pistoning, rapid and violent changes in the lake level.

Benoît has witnessed that behavior in other volcanoes, and believes it's driven by dramatic variations in magma pressure.

(lava booming) It's also been reported before-- previous eruptions from Nyiragongo.

SMETS: It's not about having the lava lake level high or low, it's understanding these movements to predict a big event, like a flank eruption, for example, because that's what happens before the last two flank eruptions.

So we had big movements of the lava lake, and all this may say something about an upcoming eruption.

♪ ♪ NARRATOR: Benoît's cameras are capable of spotting these abrupt changes, but they haven't yet been designed to be left in the crater and watched remotely.

So the team needs to find another way to warn the people of Goma of an impending eruption.

Volcanologist Jeff Johnson thinks he can do this by listening to the sounds Nyiragongo produces-- but not just any sounds.

JOHNSON: This is a custom-built microphone, and it's capable of recording sounds beyond the threshold of human hearing.

NARRATOR: Jeff's microphone is designed to pick up very low frequency sound-- what's known as infrasound.

Volcanoes speak at low frequencies.

They generate sounds that we can hear, but they also generate this world of infrasound, a unique voice print that we want to recognize and understand, so that when that tone changes in the future, we will be able to understand what's going on.

NARRATOR: If Jeff is right, then like an organ pipe, this tone will change as the level of the lava lake rises and falls.

Okay.

NARRATOR: Using sound in this way has great potential, and Chris Jackson is eager to see how it works.

JACKSON: So we're listening to sounds coming from the lava lake, is that right?

We're trying to hear the lava lake with these sensors.

JOHNSON: The infrasound is detecting motions that occur both at the lava lake surface and also inside this bowl that could be vibrating.

You don't think of a caldera this big as being an air mass that may be going up and down.

JACKSON: No.

JOHNSON: But that's what we have discovered-- the crater actually acts as a musical instrument.

NARRATOR: First, the scientists need to install the microphones.

To detect this low-frequency sound, Jeff and Chris place groups of sensors at several locations around tier two of the crater, as close to the edge as they dare.

JOHNSON: Go from here to there.

JACKSON: Yep.

JOHNSON: And from here to there.

JACKSON: Okay.

♪ ♪ NARRATOR: And it's not long before they're getting results.

JACKSON: So we've collected some data.

It looks like a bunch of wiggles on a screen to me.

But what noise is the volcano making?

Right, so we can't hear infrasound, but we can speed it up, and we can make it audible.

Here's an example of the infrasound being sped up by a factor of 40.

(recording rattles) This to me is exciting.

I see the data; it's good, good quality, and I am happy.

♪ ♪ NARRATOR: Even more exciting is the possibility of leaving a network of microphones in the crater to detect changes in the sounds the volcano is making.

On its own or with Benoît's images of the lava lake, this data could give scientists precise clues to the volcano's behavior, and for the people of Goma, it could lead to better early warnings of an eruption.

JACKSON: So it would be fair to say that infrasound could help better protect the people of Goma from a volcanic eruption?

So, I'm a scientist, and I'm naturally cagey about answering a question like that, but yes, I do believe that infrasound is a fundamental tool for volcano monitoring, and not too far down the road, we will be able to use infrasound monitoring here to better forecast Nyiragongo's next eruption.

♪ ♪ NARRATOR: Up on the volcano rim, Kayla has yet another idea for an early-warning system.

She studies the gases that constantly vent from the lava lake's surface, because she thinks they can tell her what's happening in the magma chamber.

IACOVINO: The real power in gas measurements is that it can tell us about the entire system miles and miles beneath your feet.

That's where the action is, that is the driving force of volcanism.

It's controlled deep down in the guts of the volcano.

NARRATOR: All lava contains gases, but when an eruption is building, those gases change.

The most worrying gas is sulfur dioxide, because an increase often signals that lava is moving up towards the surface.

By placing a device called a gas box where the volcano vents, Kayla can measure the amount of sulfur dioxide coming off the lava.

IACOVINO: Sulfur dioxide is the kind of gas that bubbles out of the magma in the really shallow part of the system, so just beneath the lava lake.

If we see, all of a sudden, a huge spike in the amount of sulfur dioxide that's coming out of the crater, that could be something that happens before an eruption.

♪ ♪ NARRATOR: After 12 hours, Kayla returns to the vent to find out what's been recorded.

IACOVINO: So I'm just looking at the data now, and I'm pretty happy.

So these are sulfur dioxide, we're getting some readings there.

Less than one part per million, but there is some reading there, and it's something that we should keep an eye on to, to try to predict future activity.

♪ ♪ NARRATOR: Kayla sees this low reading as good news, suggesting new magma is not rising up through the volcano.

For the moment, Goma appears safe.

But sampling gases is unreliable, because it depends on weather conditions and wind direction.

Perhaps a combination of early-warning techniques is needed to protect Goma, but no warning system is perfect.

So one key question remains: if the volcano starts erupting without warning... How long will people have before the lava reaches the city?

That depends on its chemical properties, which determine how quickly it flows.

To estimate the speed that lava will travel during the next eruption, the team needs fresh samples.

Ideally, they would take samples directly from the lake, but that's simply too dangerous.

There is, though, another possibility.

During preparations for the descent, Aldo witnessed a new vent opening up... Holy (bleep).

NARRATOR: ...that sent rivers of lava running across tier three, the crater floor.

KANE: So that's supposed to be heading down, but this aggressive vent here is, is constantly boiling and the...

I mean, you can see there the lava bombs that are getting blown out of there are probably 40, 50 meters into the air.

♪ ♪ NARRATOR: For Aldo, it was terrifying.

There's just so many, uh... NARRATOR: Now the vent is simply smoking, but the lava flows it left behind could be what the scientists need.

So the team plans to descend to tier three.

First, though, they need to check that the lava has cooled enough to walk on.

JACKSON: We're about to launch a thermal camera fitted to a drone that the Belgian science team have brought along, and it's going to be flown over T3, the, um, lowermost level next to the lava lake, specifically to look at where there may be hot rocks or magma underneath the thin crust.

If you go over the front ones, we know they are about 60 degrees, so this would be, like, something we can use for T3.

What's his max?

500.

500?

Yeah, 500.

Don't go over the lake!

No.

(drone buzzing) KANE: He's flying back this way along.

OLIVIER NAMUR: There's, there's something really hot there.

Could be this vent?

SMETS: I have a problem with the drone.

I cannot control it.

Is the drone, have you got control?

No.

Maybe you can move, try to keep the signal... Yeah, I think here, if you just watch the...

Okay.

So we're kind of almost over the area that we'll be running the ropes in and abseiling down, aren't we?

Was that safe enough for tomorrow, that we... NAMUR: I think that's fine, as long as we don't go too close to the vent, which was really hot, but everything else was okay.

I'm coming back, because I cannot control the drone.

Okay, I can see it.

The wind is too strong.

I've got visual.

(drone buzzing) ♪ ♪ NARRATOR: The drone's thermal camera shows that the place the team wants to collect samples from is about 140 degrees Fahrenheit, not too hot to walk on.

At the edge of the cliff face, Aldo starts preparing the climb down for Olivier Namur, who studies the chemical composition of rocks.

He thinks samples from the crater floor will reveal that the next eruption will consist of fast- or slow-flowing lava.

I'm interested in the composition of the lavas and the evolution through time.

So I've been sampling old lavas in the last couple of days.

And I will be sampling these very young lavas that erupted last year on tier three.

KANE: I think it's round about a hundred meters, so that's...

I think where we are now is about the height of the White Cliffs of Dover?

MAN: Yeah, thereabouts.

NAMUR: I've never been down here before.

This is gonna be my first time.

NARRATOR: It isn't a straightforward descent.

There's an initial 90-foot climb down, then a sloping field of fallen boulders, where the crater wall has collapsed, followed by a final vertical drop to the crater floor.

It all has to be rigged safely, so Aldo and his climbing partner Daz descend first.

KANE: So brittle.

MAN (on radio): Go ahead.

That's both Daz and I on boulder field, over.

KANE: Holy (bleep).

Whoa, there are some big chunks of rock there, mate.

About 150 meters away from the lava lake at the minute, but I reckon, Daz, about 80 meters, so it's about 80 meters straight down there.

♪ ♪ NARRATOR: At the foot of the cliff, there's evidence of a dangerous recent rock fall, and up on the boulder field, a sudden plume of sulfurous gas means Aldo has to wear a breathing mask.

Oh, man, look at these rocks.

Just precariously balanced.

If any of these rocks decide to go... Then that's it.

♪ ♪ NARRATOR: When Aldo drops over the edge, Daz stays up on the boulder field to keep an eye out on the rock face.

PHILLIPS (on radio): Yeah, I know, I can see this bit, across to our left-hand side as you're climbing, looks right dodgy.

NARRATOR: As he descends, it's clear any false move could create a lethal rockfall.

KANE (on radio): There's stuff under here, mate, the size of minivans, just hanging on by a thread.

NARRATOR: Even up on the camping level, Benoît can see the risk Aldo's taking.

SMETS (on radio): There are big rocks above you, uh, on the left.

They look pretty scary.

PHILLIPS (on radio): Nice, got you.

So I've just arrived on tier three.

The lava lake is about 100 meters that way, and, uh, that is the route that I've just abseiled down.

It is, without a doubt, one of the most dangerous things I've ever done.

My mouth is dry.

Ooh.

And my heart rate is up.

All the classic signs of... 100%, pure, unadulterated fear.

(sighs, equipment beeps) (breathing heavily) NARRATOR: All the classic signs that it's time to climb back up.

Um...

It is super-sketchy.

I think it's the most sketchiest thing that... that I've seen since being in here.

I don't know what you're used to, but...

I'm not entirely sure I would go back down there.

If you think it's not a good idea, then not take a risk.

I mean, we are here to do good science and collect exceptional data, but not taking stupid risks.

I know you well enough to know that if that, uh, situation down there is, uh... You're fearful of that, then... No, it's too dangerous.

Okay.

Let's forget about going to tier three.

Okay.

NARRATOR: The descent to the crater floor is abandoned, but there is still another possible source of fresh lava.

The vent activity Aldo witnessed during preparation threw out chunks of solid lava, or lava bombs, and some may have landed on the boulder field halfway down to tier three.

If they can be found, the science team will have samples of fairly fresh lava.

For the rock samples, we can have some spatters... Yeah, but only from the... SMETS: Coming from the vent in the boulder field.

KANE: It's not ideal.

It's not ideal, but it's better than nothing.

It's the best we can do.

♪ ♪ NARRATOR: The team descends quickly.

No one wants to hang around here too long.

KANE: I mean, you're standing, hammering a cliff which is clearly already unstable.

Yeah, this is true, but this is the only way to get these samples.

NARRATOR: Olivier soon finds what he was looking for-- new lava bombs.

KANE: What have you got?

It's a very fine-grained lava.

This should be enough.

It's quite fresh.

I think they will tell us quite a lot about the recent activity of the volcano.

(talks softly) ♪ ♪ NARRATOR: After an uneventful hour, he has enough samples.

For a volcanologist, they contain an unmistakable message.

Let me show you one of the samples that I collected from the active vent.

We can see that, uh, this sample is a glassy black matrix.

We can see a lot of bubbles here around and few, um, tiny white crystals.

We know that the composition of this volcano are low in silica, very low, below 40%, and this makes this lava very fluid.

So they have low viscosity, they will be flowing like water along the flanks of the volcano rather than mud.

♪ ♪ NARRATOR: Silica is a key component of sand, but also of lava.

The less there is, the more fluid the lava, and the faster it flows.

Nyiragongo has some of the lowest-silica-content lava on the planet.

But there's another clue to the lava's speed in Olivier's sample, and it's not good news.

And on top of that, because they have only a few crystals, that decrease again the viscosity of this lava.

But because they have only a few crystals, they are very fluid.

So I suspect that if there is a new eruption with this composition, it might be flowing even faster than during 2002.

♪ ♪ NARRATOR: In 2002, lava flowed toward Goma at reported speeds of up to 25 miles an hour.

Olivier's samples reveal that next time, it could flow more quickly.

The people of Goma will have less time to evacuate, making the need for an effective warning system even more urgent.

♪ ♪ The expedition is coming to an end.

The team is getting ready to head out of the crater.

The scientists have installed seismic stations around Nyiragongo and has tested a variety of technologies to monitor the lava lake and detect the build-up of pressure in the magma chamber below.

The task is not done, but it's a good start.

IACOVINO: Volcanoes can live for millions of years, and we're here for a couple of weeks.

But we're getting the beginnings of an idea of what this volcano is capable of doing.

NARRATOR: It will always be dangerous to live in this highly volcanic landscape.

As their work has revealed, for the moment, Nyiragongo is quiet.

But it will not always remain so.

The quest to understand this volcano and its fiery lake must go on.

(lava bubbling, rock shifting) JACKSON: Nyiragongo is not an easy volcano to study.

It is a massive headache in terms of getting people and equipment here.

The motivation for it is very clear.

There are a million people living very close to this volcano, so despite all the problems, it's worth it.

♪ ♪ ANNOUNCER: Heading into danger... MAN: Armed groups around the volcano.

ANNOUNCER: A scientific team explores a once-powerful volcano gone eerily quiet.

I got a view of the entire volcanic landscape.

ANNOUNCER: But will it roar back, deadlier than before, putting lives at risk?

We've lowered the probe in.

ANNOUNCER: Time may be running out to solve the mystery of... "Volcano on the Brink"-- coming up next.

♪ ♪ 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.

♪ ♪