♪ ♪ (thunder crashing) NARRATOR: In January 2022, the volcano Hunga Tonga- Hunga Ha'apai produces the most explosive eruption ever recorded.

(volcano erupting, people exclaiming) MOANA PAEA: It was so massive!

Threw all of us on the ground!

MAN: Look, look!

BRANKO SUGAR: Suddenly, I can see a wall of white water coming fast.

MAN: Here!

PAEA: And I just screamed, "It's a tsunami!"

SUGAR: Everyone, life jackets!

Get your life jackets!

SUGAR: I accepted the fact that we had three minutes left to live.

(waves crashing, people crying) (people exclaiming) NARRATOR: Waves as high as 60 feet devastate the Pacific nation of Tonga.

Now... Running away!

NARRATOR: ...a team of scientists tries to solve the mysteries of this disaster.

TAANIELA KULA: This is a turning point to how we understand the world.

NARRATOR: Can they reconstruct the key events...

The wave came from behind us and took out these columns.

NARRATOR: ...and answer the critical question, could it happen again?

SHANE CRONIN: Everything that we've discovered today is absolutely new, and I think will be a quite a shock.

NARRATOR: "Hidden Volcano Abyss," right now, on "NOVA."

♪ ♪ ANNOUNCER: Major funding for "NOVA" is provided by the following: (man speaking on radio) SUGAR: We were going spearfishing, just like we do now.

Me, my son, and the boys.

(man speaking on radio) The volcano's been erupting forever.

Every five minutes, for months.

But no big eruption, just... Just nice ones, beautiful to see.

We could see it from here.

It was nothing out of the ordinary.

And then we see the mushroom was kind of getting bigger.

Look, look!

It just happened in a couple of minutes, that is... MAN 2: How quick it's spreading.

MAN 1: Wow.

So the boys want to stop and take some photos.

I said, "Ah, yeah, okay."

So I stopped the boat right here.

♪ ♪ MAN: Whoo!

(all laughing) (eruption echoes, men exclaim) SUGAR: And suddenly the explosions came.

(eruptions exploding) One, two, boom, bang, bing, bong.

Four, five of them.

I realized this one is different.

MAN: Wow!

SUGAR: And suddenly, I can see white water from one end to the next, as far as you can see.

♪ ♪ A wall of white water.

Nobody think much about it, but then it hit Fafa Island.

The wave was higher than the coconut trees.

You couldn't see the island-- nothing.

I knew what was coming.

Tsunami.

I accepted the fact that we had three minutes left to live.

I just felt sorry for the boys, my son and the other boys.

They were busy taking pictures and laughing.

(men laughing) SUGAR: But I knew, you get hit by tsunami, you go to deep water.

Just go.

♪ ♪ But the wave is chasing us and coming fast.

(in video): Everyone, life jackets!

Get your life jackets!

(present day): And the sky was gray and murky.

But now we're in the deep water.

We can see the wave come to the other side and hit the main island.

It must have gone underneath us-- we didn't even feel it.

We thought we were now okay, just go home and that's it.

But the worst was to come.

(rocks, debris splashing) It start to rain rocks on us.

(in video): It's getting worse!

(present day): And then... (in video): No!

(present day): Everything went pitch black.

(debris falling, water splashing) You couldn't see your hand in front of you.

Everything was dark, no electricity on the island.

No electricity, no lights.

I look at the boys.

Everybody is silent.

Three hours, fighting the sea.

NARRATOR: Branko and his family had survived the most explosive event ever recorded on Earth.

(eruption explodes) At 5:15 p.m. on January 15, 2022, satellites witnessed the volcano Hunga Tonga-Hunga Ha'apai blast a cloud of ash and rocks up into the atmosphere.

(eruption echoes, people shouting) (water rushing) NARRATOR: Within minutes, tsunami waves crashed into the South Pacific nation of Tonga.

(baby crying) (translated): I ran with my son.

I could see that the wave was higher than this house.

(people shouting) SALOTE KAFOA (translated): We saw a huge wave.

It came splashing over the top of the church and school roofs.

(people exclaiming) (translated): A boy came and reached out to grab me.

I said, "What are you doing?"

He said, "I am going to carry you, so we can escape."

(woman speaking non-English language) (eruption echoes, woman yelps) NARRATOR: Rock and debris rained down on Tonga for over ten hours... (debris falling, clanging) WOMAN: Jesus Christ.

(child exclaiming in background) NARRATOR: ...covering the country in a layer of volcanic dust.

♪ ♪ MARIAN KUPU: It was a disaster.

It was like driving through a movie set where a bomb just exploded.

It was left with dust and nothing but-- no color.

It was just like a black-and-white movie.

NARRATOR: Journalist Marian Kupu recorded the impact on her country.

KUPU: This is my route towards work.

And I would drive here and I would take pictures.

♪ ♪ There was hopelessness in the faces of people.

(siren blaring in distance) People in shock, calling and looking for loved ones.

(talking in background) KUPU: We didn't know-- are we gonna all die?

Is this the end of the world?

♪ ♪ NARRATOR: Whole villages were destroyed.

But, remarkably, across the country, there were just three deaths.

(explosion echoes) Scientists around the world struggled to explain the enormity of what had happened.

(rumbling) CRONIN: From satellites, we saw this huge eruption.

We could not believe it.

(rumbling) And we still don't know what actually generated that tsunami.

NARRATOR: Discovering how Hunga Tonga-Hunga Ha'apai caused this disaster could transform our understanding of the threats from volcanoes.

(thunder rumbling) The Kingdom of Tonga.

Home to over 100,000 people.

It is one of the most isolated countries on Earth.

Separated from Australia and New Zealand by hundreds of miles of open ocean, it is an archipelago of over 170 separate islands in the South Pacific.

♪ ♪ Less than 40 miles from the main island of Tongatapu is the volcano Hunga Tonga-Hunga Ha'apai.

(eruption echoes) WOMAN: Oh, my God!

NARRATOR: When it erupts in January 2022, the world is in the grip of the COVID-19 pandemic.

Tonga's borders are closed, making the nation even more isolated than usual.

Local scientist Taaniela Kula is one of the first to respond.

KULA: As a geologist, I will never forget this event.

It is the turning point of how we understand the world.

NARRATOR: An immediate concern is the volcanic ash.

Over six million tons fell on Tongatapu alone... ...transforming a once-green environment.

(radios running, sirens blaring in distance) Tongans mobilize to begin the cleanup.

But Taaniela fears the ash poses a danger to human health.

KULA: Rainwater is our source of drinking water, and volcanic ash, when it interacts with water, makes the water more acidic, so it causes health problems.

So we started collecting the ash, so that we know the true contamination level.

(radio running) NARRATOR: Samples reveal the ash isn't toxic.

And Tongans can start rebuilding their lives.

For Taaniela, mysteries remain.

Why did the volcano suddenly erupt so violently?

And is it still a danger to his country?

To find out, he needs equipment and resources from overseas.

Nine months after the eruption, he gets his opportunity.

(metal clanging) The COVID restrictions have been relaxed.

Tonga's borders have reopened.

And an international team of scientists can join Taaniela on an expedition to the Hunga volcano.

KULA: People are still anxious to know what's the status at this point.

It's our responsibility to make sure we know our environment and what's the risk.

NARRATOR: On the ship are marine geologist Marta Ribó and a volcanologist who has spent decades studying the volcanoes of the South Pacific... That one just needs to be parallel with this one.

NARRATOR: ...Shane Cronin.

CRONIN: That's it.

CRONIN: I've been working with Taaniela for the last 20 years or so... All right!

...to understand just how these volcanoes work.

Let's rock!

(calls out) Ooh, I feel we're moving.

Hunga is one of these typical volcanoes that sort of puffed away for a long time-- so in this case, decades-- just producing these small eruptions, and then there was this incredibly big eruption.

This was generated by something at the volcano, we just don't know what.

NARRATOR: Ahead of them lies a five-hour voyage to one of the most geologically active places on Earth.

♪ ♪ To the east of Tonga, two of the planet's vast tectonic plates are crashing into each other.

The Pacific Plate is being forced downwards, in a process called subduction.

As it descends towards the hot center of the Earth, it gets warmer.

The rocks begin to melt, turning into liquid magma, which rises towards the surface, collecting in magma chambers under the sea floor.

Occasionally, this magma surges upwards... ...and erupts out of the volcano.

But the constant subduction refills the magma chamber.

Which can trigger more eruptions.

♪ ♪ 40 miles from Tongatapu, the ship reaches an area of open ocean.

This is the site of the volcano.

All that is visible are two small islands, Hunga Tonga and Hunga Ha'apai.

Looking behind me, you'll see, seeing the island of Hunga Ha'apai.

So that's right on the edge of the volcano.

The main volcano is actually over to the east.

So in this great big expanse of what looks just like completely clear ocean, we're actually sitting right on top of the Hunga caldera.

So below us is actually the volcano.

NARRATOR: Hunga Tonga-Hunga Ha'apai is a submarine volcano.

Its two-and-a-half-mile-wide crater sits at the top of a mountain nearly 6,000 feet tall, which is almost completely hidden underwater.

Only in two places is the crater wall high enough to break the surface, forming the two islands.

Before the January 15 eruption, the volcano looked very different.

The center of the crater was 2,300 feet shallower.

The islands were far bigger, and between them was the volcanic cone.

(eruptions rumbling) On December 20, 2021, this cone suddenly sprang to life.

Usually, eruptions in Tonga are monitored by satellite, but space is too far away to pick up all the detail.

(man speaking non-English language) NARRATOR: So, for almost four weeks, Taaniela regularly came to the volcano to check for increasing activity with a drone.

(eruption echoes) KULA: We came here on the evening of the 14th of January.

I saw a column of ash and gas... ...almost two miles wide just rising into the heavens.

(thunder rumbling) There were lightning bolts everywhere.

It happened all around the island.

(thunder rumbling) It's like shooting several thousands of crack, firecrackers all at the same time.

(eruption exploding) It felt like I'm witnessing Mother Earth's might and power being released.

(thunder crashing) NARRATOR: Eight hours later, the volcano stopped erupting.

Then, at 5:15 p.m. on January 15... ...it produced an eruption over 70 times larger... ...exploding with the power of over 60 million tons of TNT and triggering a series of tsunami waves that hit Tonga, Australia, Japan, and the Americas.

Why had this volcano suddenly produced such an immense explosion?

Why was it so much more powerful than the previous day's eruption?

And is the volcano active enough to erupt again in the near future?

First, the team needs to know the volcano's current activity.

CRONIN (chuckling): The weather's not ideal.

I mean, the big problem for us is, is the sea condition, and a strong wind is on its way.

So we're really hoping that we can get all the survey done before that, that starts coming on.

(radio running) NARRATOR: Marta is trying to gauge the volcano's activity using sonar.

(pinging faintly) The echo sounder is sending a ping down, and the reflection of that ping noise, it's coming back and it's building up this 3D model of the sea floor.

NARRATOR: The scanning will reveal if magma is dangerously close to the surface.

♪ ♪ (people talking and laughing in background) Wow, these are, it's stronger than the one before.

CRONIN: So these are the shallow ones... NARRATOR: Escaping from the sea floor are plumes of volcanic gases released by the magma.

1,340 meters.

NARRATOR: The closer the magma is to the surface, the hotter the plumes and the more likely an eruption.

(talking in background) CRONIN: This is a sensor, which actually measures, 2,000 times a second, the temperature of the water.

Is the volcano heating the water, for example?

Is the volcano changing the water temperature?

It's all the question of whether the volcano is alive or not.

That's it, that's it, that's it!

RIBÓ: Careful.

CRONIN: Down, down, down, quick.

Keep going, yup.

NARRATOR: Measuring multiple plumes will reveal how close the volcano is to erupting.

Running away!

(waves crashing, man exclaims) KULA: It's a big one!

NARRATOR: They see a consistent trend.

RIBÓ: All of the plumes, they're very weak.

NARRATOR: The plumes aren't hot.

RIBÓ: It's still active, but the strength of the volcano has decreased.

So for the people of Tonga, there is still some risk, but it's less dangerous.

NARRATOR: The team's findings suggest the Hunga volcano won't produce another massive eruption-- for the time being.

But the events of January 15 show this is an unpredictable volcano.

Why did it suddenly produce an eruption 70 times bigger than the previous day?

It's a question with global implications.

There could be as many as 50,000 submarine volcanoes lurking under our planet's waves.

In recent years, Kolumbo, near the Greek island of Santorini, and Oomurodashi, less than 40 miles from the center of Tokyo, have been identified as active threats.

Knowing exactly how the Hunga volcano generated such a powerful eruption could change our understanding of the dangers from submarine volcanoes.

(debris falling, people shouting) And the clues could lie in the ash Hunga Tonga-Hunga Ha'apai rained down on the islands.

♪ ♪ The debris contains time capsules from the eruption, which volcanologist Joali Paredes-Mariño analyzes.

The first thing that highlights, to, to me, is this very dark and, like, glossy, blocky particles-- so, that's glass.

NARRATOR: Volcanic glass is solidified magma, created at the very moment the volcano exploded.

A powerful electron microscope allows Joali to compare different samples of glass.

This one's from a different eruption.

We got this kind of, like, massive glass, covered with bubbles.

MAN: Right.

NARRATOR: Explosions happen when a material expands very rapidly.

In most eruptions, this material is gas in the magma.

Normally, this gas is dissolved in the liquid magma, just like in a soda.

If some reason, the pressure that is on that magma change, like when you open a bottle of soda, you will have all these bubbles.

They will create this inner force that, at the moment that it cannot hold it any longer, it will fragment the magma.

And that's what creates an eruption from within.

NARRATOR: The growing bubbles of gas inside the magma cause it to expand rapidly and explode, and are clearly visible in glass from a typical eruption.

But the team sees something very different in this sample from the Hunga volcano.

If we see the one in the center, you don't see those bubbles.

They have kind of blocky particles.

NARRATOR: No bubbles means the eruption wasn't generated by the sudden expansion of gas in the magma.

The shapes of the fragments point to the explosion of a completely different material.

You have this kind of, like, step fractures.

And also, you can see the big particle in the bottom corner.

This is concave fractures.

NARRATOR: This pattern of fractures is consistent with a very specific explosive event.

That means that is telling us this is interaction with magma and water.

NARRATOR: Water can be extremely explosive.

If it hits something as hot as magma, water instantly turns to gas, expanding as much as 4,000 times.

(thunder crashing) It would seem the most intense eruption ever witnessed by science was detonated by the explosion of water.

♪ ♪ But there is a mystery.

The volcano's magma is usually locked away from the ocean, in a chamber over three miles below the surface.

Why did water and magma suddenly come into contact on January 15?

In the global seismic records, the team spots signals the satellites didn't detect.

Okay.

That's it?

Uh-huh!

Yes.

NARRATOR: 28 minutes before the big eruption, there was a series of smaller, precursory eruptions.

CRONIN: We think the precursory eruptions led to the rapid fracturing of the upper volcano.

And then seawater gets down into the magma, and away we go, and we start a very, very violent explosion.

NARRATOR: The magma and water were separated by thousands of feet of solid rock.

The team thinks, on January 15, the first of the precursory eruptions cracked open faults.

As this series of eruptions continued, these cracks grew bigger.

And bigger.

Until finally, they allowed cold seawater to flood in, hitting incredibly hot magma in the chamber and generating the immense explosion.

(booming) The investigators have made a major breakthrough: an explanation for how the Hunga volcano could have produced such a powerful explosion.

But they face another problem.

(eruption explodes, thunder crashes) Most tsunamis are caused by earthquakes.

Volcanic tsunamis are so rare, scientists don't know how the Hunga volcano generated a series of enormous waves.

Discovering what happened here could give new insights into the global threat from submarine volcanoes.

The answers could lie on Tonga's Hihifo peninsula.

Once, this area was dotted with thriving resorts.

One of the busiest was owned by Moana Paea.

PAEA: The whole place was just full of beautiful big trees and coconut trees and palm trees.

You wouldn't have seen the ocean.

That used to be the tree with all surfboards on it.

It was a massive tree.

As a kid, we would climb around it.

But after that tree, there was a house there, and then from there would be the beach area.

NARRATOR: The Hihifo peninsula lies on the northwest coast of Tongatapu.

Less than 40 miles due north is the volcano Hunga Tonga-Hunga Ha'apai.

(volcano rumbling, thunder crashing) PAEA: The volcano was going crazy, striking lightning and stuff.

Because it'd been happening for weeks and weeks, we kind of took it for granted that it must be all right.

♪ ♪ We had a full hotel, and maybe quarter past five, I heard screaming.

MAN: Here!

PAEA: One of my staff said, "Hey, look at the water."

It looked like a washing machine.

(man exclaims) And I just screamed, "It's a tsunami!"

(people yelling in background) PAEA: And people were going to pack their clothes, and I said, "No, you need to get out!"

One family literally just got out of the pool and got in their car and took off.

(men speaking indistinctly) PAEA: That's when I saw... MAN: Oh, my God.

PAEA: ...the water is moving fast, but growing.

So we ran through here.

(people exclaiming) WOMAN: Oh!

Oh, (bleep)!

PAEA: We were yelling, "Tsunami!

Tsunami!"

MAN: Holy (bleep)!

(people exclaiming) PAEA: And my son was, like, "Mom, let's climb a tree."

And then, suddenly... (explosion pounds) MAN: Oh, (bleep)!

WOMAN: Whoa!

MAN: Holy (bleep)!

PAEA: Another massive explosion.

MAN: Tsunami's heading in.

(people exclaiming) (bleep) The pressure of it threw all of us on the ground.

My daughter was crying, "My ears!"

But my sons are, like, "Mom, get up, get up!"

Because we could still hear waves rumbling.

(men shouting indistinctly) MAN: Okay, okay!

(baby crying) (man panting) MAN: Here!

(waves crashing, water rushing) PAEA: And when we got down to the road, there's this two-story house there, and there were all these village people up on top of there, and it started to rain down with dust.

(baby crying, people murmuring and exclaiming) (water rushing) PAEA: When we got to the roof, we saw the big swell coming across.

(people whimpering and shouting) PAEA: We were praying and singing.

And then my brother came, so we all came down.

(water rushing) We got in the car, and it being so dark, and the dust was massive, and rocks coming... (debris pelting) CHILD: Mommy... PAEA: And they weren't just little rocks.

(debris pelting) (people whimpering, murmuring) ♪ ♪ (people speaking in background) NARRATOR: Like many Tongans, Moana headed for the safety of high ground.

♪ ♪ The next day revealed the scale of the destruction.

♪ ♪ PAEA: I remember one of my staff ringing up the next morning.

(voice breaking): And he's saying, "Sorry, Moana, everything is gone."

And I said to him, "Praise God, "we are all still alive.

That's the main thing."

(sniffs) (breath trembles) ♪ ♪ We loved it here.

Such a beautiful space.

(gasps) It was very hard to see, because this is our livelihood, and this is something that has looked after... (sniffles): ...our community for many years.

(sniffles, breathes deeply) ♪ ♪ NARRATOR: Ahead of Moana lies months of rebuilding.

This is the stark reality of the tsunami that hit Tonga.

But this event is also a mystery.

How did the volcano generate such destructive waves?

Tsunami scientist Jose Borrero has flown in to investigate.

(wind blowing) (shutter clicking) BORRERO: As tsunami waves propagate through the ocean, their speed is governed by the depth of the water.

So, the deeper the water, the faster they can go.

Now, where we are, on the west coast of Tongatapu, there was a very narrow fringing reef, and you can see the reef here.

It was where the waves are breaking.

Just on the other side of that reef, the water drops off to about a depth of just over 1,000 meters.

It's deep water from here out to the Hunga volcano.

♪ ♪ The tsunami came directly in here, unimpeded.

There was no reefs or features or anything in the way to slow it down, redirect it.

♪ ♪ And the tsunami takes only eight minutes to get from the volcano to here.

♪ ♪ NARRATOR: A wave with immense power arrived at the peninsula and caused widespread damage because of what happens when a tsunami enters shallow water.

♪ ♪ The energy of a tsunami is spread throughout the depth of the ocean.

In deep water, this makes the height of the wave barely noticeable.

But as the wave reaches shallow water, it is slowed dramatically, and the water begins to pile up, making the wave higher and higher.

BORRERO: When a tsunami comes ashore, it's not just the water that causes the damage.

It starts to pick things up.

It starts with boulders and sand.

And then it knocks down trees, and the trees become entrained in the flow.

Then it'll take out a house, and the material from the house will become part of the flow.

And all of this material gradually builds up until it's, it's not even water anymore.

It's a dense debris flow of everything that the wave has picked up along the way.

♪ ♪ NARRATOR: Jose's first task is working out the height and power of the incoming wave.

And the trail of destruction offers him the vital clues.

This is weird.

This stuff is very difficult to bend with your hand, but the way we see it here, just wrapped around a concrete pillar like tissue paper, but with roofing steel.

NARRATOR: First, Jose needs to find the wave's direction.

Over here, we see these concrete columns are knocked down in the direction the wave was going.

So the wave came from behind us, from the north, swept down the beach, and took out these columns, laying them down that way.

NARRATOR: Following the trail of debris inland, Jose can find the evidence he needs.

♪ ♪ BORRERO: Up here is one of the most crucial bits of information from the whole tsunami event.

This was a cell phone tower.

And on the cell phone tower was a weather station, and, as you can see, was completely obliterated by the tsunami surge.

This is a very strong piece of equipment.

It's a big, strong thing.

This was a 20-meter-tall tower.

But yet the force was able to bend the entire thing over.

And 200 meters inland, on one of the trees is pieces of this tower still hanging from a branch five to six meters above the ground.

NARRATOR: Bringing all of this information together, Jose can compute the scale of the tsunami.

♪ ♪ BORRERO: So the ground level that we're standing at is about 13 meters above sea level.

And then with the flow marks around this area, we know that the tsunami flow depth through here was somewhere to five to six meters.

So we're talking, you know, 18, 19 meters of, of total tsunami height passing through this area.

NARRATOR: 19 meters is over 60 feet-- about the height of a six-story building.

(water rushing) NARRATOR: In 2011, a massive earthquake unleashed a tsunami on Japan.

(people screaming) NARRATOR: Despite being protected by a breakwater costing some $1.6 billion, Kamaishi City was hit by a wave that measured 26 feet tall.

(water rushing) All of this damage was caused by a wave less than half the height of the Hihifo tsunami.

♪ ♪ How did the Hunga volcano generate such an immense wave?

(men exclaiming) BORRERO: So, yeah, this is you guys parked up here.

Yes.

NARRATOR: To find out, Jose gathers eyewitness testimony and video.

MAN 2: I was the driver of that car.

BORRERO: Were you waiting for someone to come?

No.

We were waiting because the...

The road!

Over the road.

The road was busy.

(laughs): And the waves began banging on the car.

(all exclaiming in video) (video rewinding) NARRATOR: Jose is combining this firsthand evidence with images and videos he has found on social media.

BORRERO: It's astounding, the amount of material that's available online.

People just post things up, and all of this is completely useful in a scientific sense.

We have a set of videos that show a series of tsunami waves, and we know what time the video was taken.

And by knowing the timing of when it's hitting the coast, it gives us clues as to how that wave was generated at the volcano.

NARRATOR: Jose has built a timeline of the day.

(eruption explodes in distance) MAN 1: Oh!

MAN 2: Whoo!

MAN 1: Holy (bleep)!

NARRATOR: He's identified not one, but two tsunamis-- one at 5:23 and one at 5:34.

(mouse clicking) Tracking back eight minutes from each, he finds the same thing.

An eruption at the volcano.

♪ ♪ This fits with one theory of how submarine volcanoes produce tsunamis.

As material erupts upwards, it pushes water out of the way, creating immense waves that radiate out from the volcano.

But Jose doesn't think these eruptions can explain the destructive 60-foot-high tsunami.

♪ ♪ BORRERO: We know the weather station transmitted its last packet of data at 6:00 p.m.

So we know a tsunami could not have destroyed the weather station and the cell phone tower that it was attached to until sometime after 6:00 p.m.

This means that there was a later wave.

But this doesn't fit with the idea that the wave was generated during these two volcanic eruptions.

♪ ♪ NARRATOR: If it wasn't an eruption, what other event at the Hunga volcano would have enough power to unleash a 60-foot-high tsunami?

(eruption thundering) (roaring) One possibility lies with the speed it threw out material.

CRONIN: During the first one-and-a-half hours, it was ejecting material at a rate twice as much as the next-biggest eruption that we know of.

It threw out seven cubic kilometers of dense rock material.

NARRATOR: That's enough to cover the entire island of Manhattan in nearly 400 feet of debris.

CRONIN: When a large amount of magma erupts from a volcano, large amounts of magma come out at once.

And so that means that the top part of the volcano actually then sinks in on itself.

NARRATOR: Shane's idea is that as material erupted out of the magma chamber, it created a larger and larger void underground.

When it could no longer support the weight of material above, the entire caldera dropped over 2,300 feet.

Seawater flooded in, kickstarting tsunamis that radiated out from the volcano, an explanation of how the volcano could generate a 60-foot wave.

♪ ♪ The team has found that a caldera where the water can get into the magma chamber can erupt explosively and generate life-threatening tsunamis, heightening the need to monitor these types of volcanoes.

And only 55 miles from Hunga Tonga-Hunga Ha'apai is its volcanic twin: the island of Tofua.

Just like the Hunga volcano, Tofua has a two-and-a-half- mile-wide caldera.

But its magma chamber is far closer to the surface, and far closer to the surrounding seawater.

♪ ♪ KULA: Captain, we're keeping a straight line.

We're about here.

CRONIN: It's a very remote part of the world, and so no one's been able to get here.

It's 14 hours on the boat, and out here, the ocean can be extremely treacherous.

♪ ♪ All previous measurements have been made from satellite observations, so it's critical that we try to land on the island.

NARRATOR: The satellites suggest there is virtually no activity at Tofua.

But this has never been verified with field measurements.

NARRATOR: Taaniela and Shane enlist the help of boatmen from the neighboring island of Kotu.

(motor humming) They report having seen a red glow above Tofua at night.

♪ ♪ KULA: I feel excited to go up for the first time.

We hope to get some real data on the ground.

And then, uh, we can share it with the world.

♪ ♪ NARRATOR: First, they need to ascend over 1,500 feet through tropical forests.

♪ ♪ CRONIN: It's quite humid.

So...

I think my sweat is on top of my sweat, building up a third layer of sweat.

(grunts) ♪ ♪ NARRATOR: Their first objective: the rim of Tofua's two-and-a-half-mile-wide crater.

CRONIN: Look, look, this is a big caldera.

Yeah, so there's the active cone.

That's our destination, should we decide to accept it.

NARRATOR: Shane suspects the satellites are wrong and Tofua could be due an eruption.

CRONIN: We had a big eruption around 1,200 years ago, another big one around 800 years ago, and the most recent one around 400 years ago.

NARRATOR: This pattern suggests Tofua should erupt again, and soon.

(camera shutter clicks) CRONIN: Okay, it's fantastic.

We've got a bit of a break in the weather.

You can see how blue that gas is coming off that volcano.

When the color of the gas is blue, it's sulfur dioxide.

NARRATOR: This gas is released by the magma.

The blue color is only visible if the levels of sulfur dioxide are high.

CRONIN: So I think the magma must be very close to the... (shutter clicks) ...close to the surface, actually.

(shutter clicks) And this corresponds to what the locals have been reporting over the last couple of months, is that there is a glow in the sky above Tofua when there's a cloudy evening.

NARRATOR: The signs point to Tofua being far more active than the satellite readings suggest.

But to know the level of the threat, the team needs more evidence.

Taaniela will use a drone to investigate the active cone.

While the rest of the team will collect vital readings, which need to be taken from inside the caldera itself.

♪ ♪ CRONIN: I'm going to try and get a position close to the plume.

I don't want to go underneath it, because sulfur dioxide is actually quite lethal.

We can then try to measure some of the output.

This is what I've been lugging up the hill.

NARRATOR: Volcanologists monitor the sulfur dioxide to gauge the activity of volcanoes.

CRONIN: This is the telescope.

And it's a rotating one.

NARRATOR: The more sulfur dioxide Shane detects, the more active the volcano.

Now...

It's going to be scanning over there, and I want it to scan above the level of the opposite caldera rim.

We're capturing it beautifully with this angle.

♪ ♪ NARRATOR: Shane's readings indicate the satellites have been dangerously underestimating the levels of sulfur dioxide.

Tofua is releasing far more than previously thought.

♪ ♪ Taaniela's expert flying reveals that the magma has welled up in the crater, creating a lava lake, one of fewer than ten on the entire planet.

And there's a sign Tofua has been actively throwing out lava, incredibly recently.

CRONIN: This erupted in the last few days.

It's a fresh volcanic bomb.

So this really contrasts to all of the rest of the older material around here.

So there's been some quite big explosions to produce these.

It's really interesting, because it looks as if Tofua is entering a new phase of activity, one that we haven't seen here before.

♪ ♪ Everything that we've discovered today is absolutely new, and I think will be quite a shock around the globe.

NARRATOR: Tonga is still recovering from the eruption of the Hunga volcano.

Now scientists have discovered that its volcanic twin, Tofua, is far more active than satellites have led them to believe, a finding that has implications far beyond Tonga.

♪ ♪ KULA: Our latest expedition really showed us there are limitations of the satellite system.

You always need to be on-site to actually know the full level of the volcanic activity.

NARRATOR: With as many as 50,000 submarine volcanoes around the planet, the world needs to find better ways of keeping an eye on these destructive forces of nature.

♪ ♪ And be ready for the next Hunga Tonga-Hunga Ha'apai.

(people exclaiming) (speaking Tongan): NARRATOR: Taaniela and his colleagues have long reached out to people of Tonga.

ANA MAEA (speaking Tongan): PUPILS: KULA: We feel that it's our responsibility to build awareness about geological hazards and risks that we are exposed to.

NARRATOR: It's a strategy that the rest of the world could learn from.

MAEA: NARRATOR: Awareness is a major reason just three people died in Tonga during the Hunga volcano disaster.

BORRERO: The death toll was so low because of the knowledge of tsunami.

Everybody moved to high ground, they alerted their neighbors, they alerted their friends, and everybody evacuated.

It's miraculous, but it's doing the right thing at the right time.

♪ ♪ KULA: Our existence today is a demonstration of our resilience.

♪ ♪ And I think knowledge will increase the resilience of the people in the world.

(crowd cheering) NARRATOR: Being ready is Tonga's lesson for the rest of the planet.

(cheering) ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪