NARRATOR: Explosions.

Earthquakes.

You guys all right?

MAN: Would you get away from there?!

NARRATOR: Lava in the streets.

MAN: This is insane.

NARRATOR: In 2018, Hawai'i experiences its most destructive volcanic eruption in over a quarter-century.

MAN: Whoa!

NARRATOR: Residents flee for their lives.

WOMAN: We're packed, and we're evacuating.

NARRATOR: As the Earth erupts beneath their homes.

(women screaming) WOMAN: There was a house there.

Yeah, it's gone, it's under the lava.

NARRATOR: And nature turns one corner of Hawaii into a molten river.

But through it all, scientists are on the front line... (bursting) ...tracking subterranean magma flows.

BRIAN SHIRO: Think of it like a sword cutting its way through the rock.

NARRATOR: Risking exposure to toxic gases.

You can actually see all the sulfur that is actually crystallizing.

NARRATOR: And uncovering secrets held inside the molten rock.

The chemistry was extremely evolved.

NARRATOR: All to answer the question on everyone's mind: How bad will it get?

RYAN PERROY: Very quickly, it became of a scale and magnitude greater than what we were able to handle.

NARRATOR: "Kilauea: Hawai'i on Fire."

Right now, on "NOVA."

♪ ♪ MAN: Whoa, watch out!

NARRATOR: In 2018, in the Puna District of Hawai'i, locals rush for their lives as red-hot lava bursts from under their houses.

MAN: This is-- I don't know, man!

NARRATOR: Fountaining hundreds of feet into the air.

(woman calling out) MAN: We need medical assistance here!

NARRATOR: It's an eruption on a scale not seen for a generation.

450 earthquakes in just the last 24 hours.

NARRATOR: Upending the lives of thousands and transforming the landscape of Hawai'i.

But while residents evacuate, scientists head into the field, tracking the action to discover what may be next.

How accurate are their predictions?

We knew it was not if, but when.

NARRATOR: Is this the start of a new period of dangerous volcanic activity?

STACY WELCH: This is unstoppable.

This is epic.

NARRATOR: And what does it tell us about the true nature of the iconic islands of Hawai'i?

(rumbling) 2,400 miles to the southwest of the U.S. mainland lies the Hawaiian archipelago, a chain of tropical islands covered in lush rainforest and home to vibrant wildlife.

Its natural wonders make it a hotspot for tourists seeking sun and surf.

But none of Hawaii would exist without volcanoes.

(water splashing) On the southern part of the southernmost island lies Kilauea, an active volcano within the Hawai'i Volcanoes National Park.

Lava is visible in a small lake at the summit and at Pu'u 'O'o, a seeping vent on the flank of the mountain.

Both sites are popular with tourists and constantly monitored by a team of on-site volcanologists.

But in April 2018, the scientists of the Hawaiian Volcano Observatory become concerned about rising levels of lava.

We had a situation where the lava lake at the summit was high, higher than it had been for quite a while, and in fact it was periodically beginning to overflow.

At the same time, the lava level within the Pu'u 'O'o vent was also very high.

One of the things we've learned is that the magmatic system, the plumbing system is connected.

NARRATOR: Magma is underground lava.

Both lava lakes are fed from the same deep magma chamber lying miles below the summit.

They act like pressure gauges.

The recent high levels of the lakes are an indication that magma is building up under the volcano.

NEAL: The magmatic pressure within the volcano was high.

This meant there could be a change coming, although we weren't quite sure the nature of that change.

NARRATOR: On April 30, that change arrives with shocking speed.

In a matter of hours, all the lava in the Pu'u 'O'o vent disappears, leaving a gaping hole.

It catches the volcanologists off-guard.

NEAL: We were a little surprised.

Past events at Pu'u 'O'o have seen collapses, short-term collapses, but none as deep as this one.

NARRATOR: Almost 60 million tons of red-hot lava have just gone missing.

The scientists have to find it, and they fear the worst when the ground begins to shake.

(rumbling) The seismograms were off the charts, and, you know, I knew something big was happening.

NARRATOR: Brian Shiro is a seismologist with the Hawaiian Volcano Observatory.

He monitors a network of around 80 earthquake detectors positioned around the volcano.

When Pu'u 'O'o's lava disappears, the network lights up.

Around the vent, the land is being shaken by a swarm of tiny tremors.

A swarm of earthquakes is a grouping of earthquakes close together in space and time.

Given the context of Pu'u 'O'o, there really was no question what this was related to.

NARRATOR: Below the surface, pressure from the magma has suddenly fractured the surrounding rock, opening cracks that fill with magma, generating earthquakes and triggering more fractures.

A blade-like intrusion of magma is tunneling east.

SHIRO: Think of it like a sword that's traveling beneath the ground and cutting its way through the rock as it goes.

(rock grinding) It was moving underground at a rate probably about as fast as you could walk.

♪ ♪ NARRATOR: The earthquakes indicate the Pu'u 'O'o magma is advancing down Kilauea's east rift, heading out of the national park, down towards the ocean 20 miles away.

Directly in its path lie the homes of thousands of people.

NEAL: It was possible that magma would move through the rift zone system and not erupt, but there was also a strong likelihood that it would eventually find its way to the surface.

We decided we must inform the Civil Defense authorities.

We may not have a lot of time for warning.

(crashing) NARRATOR: For 48 hours, the earthquakes have moved east.

But suddenly they stop.

The magma is changing direction.

If the magma stops moving sideways, yet there's still pressure pushing it, it's going to have to go somewhere.

NARRATOR: The magma is creeping upwards.

Above lies Leilani Estates, a sleepy neighborhood about to have a rude awakening.

The local Civil Defense agency puts everyone on alert.

MAN (on radio): This is a Civil Defense message.

An eruption is possible.

BILL HANSON: We were getting reports that there was a lot of earthquake activity.

MAN (on radio): Prepare in case of the need for evacuation.

We were hoping for the best, but we were planning for the worst.

NARRATOR: As the ground continues to shake, residents soon see their neighborhood is in trouble.

WOMAN: Here is the crack, just right after the community center and the playground.

I don't know if you can see that.

There's my hand.

NARRATOR: The cracks grow in size and number.

WOMAN: This is new, coming out of there.

NARRATOR: And steam begins to billow from the depths.

At that point, we were fairly certain.

We knew that it was not if, but when.

NARRATOR: At 5:00 p.m. on May 3, the ground opens and the nightmare begins.

(barking) (bang echoes) WOMAN: Oh, my God.

We're packed and we're evacuating.

NARRATOR: The eruption quickly becomes international news.

A part of Hawai'i is under a mandatory evacuation.

This right here is a residential area about 30 kilometers from Mt.

Kilauea, and lava is being thrown up in the middle of the road.

Authorities have ordered more than 1,700 people to leave immediately.

I heard this roar.

Almost like a fire-breathing dragon sort of a noise.

It's the birth of a fissure.

(banging) NARRATOR: One by one, fissures are opening up.

Over the next few days, the neighborhood is ripped apart.

(banging) MAN: This is freakin'-- I don't know, man.

(weeping): I've been crying all morning.

(lava churns) NARRATOR: Lava is now pouring over the land... (hissing) ...generating multiple flows that advance across roads... MAN: Would you get away from there?!

NARRATOR: ...and into homes.

(gate rattling) They are slow-moving, but relentless.

What a flow doesn't bury, it sets on fire.

(fire crackling) Even as it blackens and solidifies, it's over 1,000 degrees Fahrenheit.

MAN: Burning its way through the pavement.

(fire crackling) NARRATOR: Across the neighborhood, residents are evacuating.

REPORTER: When you finally do get back home, what will you find?

(chuckling): Ashes.

It's ashes.

♪ ♪ NARRATOR: It's now over a week since the fissures opened, and the eruption continues.

To gauge the extent of the lava flow, volcanologist Sam Mitchell from the University of Hawai'i is going in for a closer look.

It's really quite something, to be able to watch it, and especially to see it from up above.

♪ ♪ NARRATOR: 15 fissures have now erupted, spewing out enough lava to cover 88 football fields.

It's clear that all the fissures originate from the same thin, deep crack in the Earth.

At the moment, all the fissures are erupting along one single line, but the lava flows can then spread laterally either side, and that's the biggest danger.

It's difficult to know which fissures will produce large flows and how far those flows will actually go.

This really is an evolving story.

(rumbling and crackling) NARRATOR: But now the flows appear to be slowing.

After a week of activity, many of the original fissures around Leilani Estates are sputtering to a halt.

Bill Hanson's Civil Defense team notices the change.

There was spattering, yes, there was noise, yes, but there wasn't any flow; the lava was very static.

NARRATOR: After a week of chaos, could the eruption be winding down?

The only way to find out is a risky scramble to sample the lava.

(splashing) Getting up-close and personal to a lava flow is not for the faint-hearted.

But for researchers like University of Hawai'i volcanologist Cheryl Gansecki, what lava can reveal about the status of the eruption is worth the risk.

We would put out tubs to try to catch some lava falling out of the sky.

You really go as fast as you can and hope you've grabbed enough material, so you don't have to do it again.

NARRATOR: Back at the lab, the investigation can begin.

The University of Hawai'i has built a database of lavas collected from previous eruptions and created a high-speed method of identification.

(rattling) Crushing samples from the new flows into a fine powder allows an analysis of their ingredients.

All lava has a fingerprint.

Subtle differences in chemical composition allow volcanologists to determine its origins.

The results are a surprise.

The lava erupting at Leilani is not the same as the lava that disappeared from Pu'u 'O'o and headed down the mountain.

But it is just like lava that they've seen before.

GANSECKI: It was very similar in character to another eruption that had happened in that same area in 1955.

MAN (on film): Two fissures, each about 300 feet long, emitting lava, extended from the Pohoiki Road into field 114.

NARRATOR: In 1955, a massive eruption shook this part of the mountain.

It lasted three months, with lava gushing from 24 separate vents.

But evidently, not all the magma reached the surface before the eruptive pressure dropped.

Cheryl's discovery suggests that the traveling Pu'u 'O'o lava encountered the remnants of the 1955 eruption, lying dormant for more than 60 years, the pressure from the fresh lava forcing it to the surface.

Because this old magma is relatively cool, on contact with air, it solidifies quickly.

But the fresh, hot lava from Pu'u 'O'o will be far more dynamic if it reaches the surface.

This eruption isn't over-- it's only just begun.

What everybody was worried about was when that Pu'u 'O'o lava made it, then the eruption scale would change.

NARRATOR: As the Leilani neighborhood waits to see what will come next, alarms are sounding at the top of the mountain, where the summit is rumbling.

Since May 2, the lava here has retreated, following the Pu'u 'O'o lava into the depths.

It's dropped 500 feet, and is still sinking fast.

Without the lava to support them, the sides are slowly collapsing, falling on top of the descending lava and generating ominous clouds of ash and dust.

Just four miles away lies the small town of Volcano.

Residents Tom Peek and his wife, Catherine Robins, are preparing for what might be coming.

(tarp rustling) As soon as the ash starts to fall, we're tearing this down.

That's a good idea.

NARRATOR: They fear not flowing lava, but a deadly assault from the skies.

As a former National Park ranger, Tom is a keen student of Kilauea's explosive history.

From all that we understand, everything is panning out very similar.

There's some differences in timeframe, but there's a very parallel kind of story developing as if this will be a 1924 eruption.

NARRATOR: In 1924, the summit of Kilauea exploded, propelling searing ash miles into the heavens and showering eight-ton rocks up to half a mile from the summit.

The explosion was probably triggered by a build-up of gas.

When the magma level drops, falling rubble will eventually block the narrow feed pipe like a cork in a bottle.

If the magma drops below the water table, steam will form in the gap.

The build-up of pressurized gas can cause violent explosions, firing red-hot rock and ash thousands of feet into the sky.

♪ ♪ NEAL: Our geophysicists were able to model at that current rate of draining of the lava lake, when the level would pass what we believed to be the level of the water table within the volcano.

So we had projected the middle of May as the likely window for onset of explosions.

NARRATOR: While residents near the summit anxiously wait for Kilauea to blow her top, back down the mountain, the Leilani neighborhood faces a new threat.

The media broadcast a dire warning.

REPORTER: Gas so toxic it's killing trees and grass.

The fire crews that are out there have reported those toxic levels; it can be fatal.

NARRATOR: Despite the lull in eruptive activity, the gas is now being detected all around the neighborhood.

Forced to wear protective breathing equipment, volcanologist Sam Mitchell has gone in to investigate.

MITCHELL: Only a few days ago, this was actively erupting lava.

You can really feel the heat coming off of some of these fissures right here.

It's really something.

NARRATOR: The air is filled with steam, and invisible toxic gas rises from the cracks.

The giveaway: yellow mineral deposits around the fissures-- sulfur.

As we walk along, you can actually see all the sulfur that is actually crystallizing out of these gases in the cracks.

See how intense the color of these crystals are.

Like, it's precipitating out pure sulfur here.

NARRATOR: The particles of sulfur indicate that the air is filled with sulfur dioxide, bubbling out from the magma deep below.

In the presence of moisture and sunlight, the sulfur dioxide forms vog, a volcanic smog containing tiny droplets of sulfuric acid.

It makes eyes burn, lungs inflame, and can even corrode metal.

At high levels, it can be deadly.

Wow!

This is really... Really, really outgassing at the moment.

NARRATOR: High levels of sulfur dioxide continue to pump into the air each day, a sign that magma is building up just below the surface.

In another part of the neighborhood, the gas has temporarily dissipated.

Stacy Welch left her house in a hurry two days ago.

The authorities have given her a few hours to enter the danger area and check on her property.

The vegetation shows the signs of sulfur poisoning.

(sighing): All my grass is dead.

It's definitely changed color since I was here two days ago.

Definitely see a difference in the landscape and everything.

NARRATOR: Fortunately, her house is still untouched by lava.

Hey, Mom, it's me.

We're okay, we're still safe.

NARRATOR: But as she makes her inspection, there are further indications that the eruption is not over.

(booming in distance) WELCH: Wow, did you hear that?

I don't know where it's at.

Just... scary.

So scary.

NARRATOR: It's time to leave.

Although the fissures here have been quiet for days, there are signs that the lull is over.

MITCHELL: I can hear these explosions happening somewhere here on the left.

There's something happening just around the corner here.

You can really see the heat that's coming off at the end of the road here.

You can see the heat waves, wow.

I think I can see a red glow just over there, on the side.

Something's on fire.

Yeah, the vegetation's caught fire.

So it's likely there's a small breakout of lava.

(crackling) This just shows that these fissures can reopen.

At this stage, we should be hesitant of actually getting any closer.

The activity is really, really picking up.

NARRATOR: Streets away, as Stacy retreats from her property, she encounters a new flow.

WELCH (gasps): Is that lava?

We were here two hours ago and it was not this advanced, so it is really moving now.

I thought it was taillights when we pulled in.

But that is definitely not taillights.

That is 100% lava.

NARRATOR: This flow is a sign that the nature of the eruption is changing.

The fountaining lava is hotter and moving faster than the old lava that erupted a week ago.

This is the fresh lava from Pu'u 'O'o that's finally reached the surface.

This eruption has just become more dangerous.

(fire crackling) WELCH: At any time, my house could go.

This is-- this is unstoppable.

This is... epic.

I'm never gonna get this close again.

I don't think I ever want to be this close again.

And this is scary and we are not safe.

NARRATOR: With lava on the move, there is just one option: get out of the way.

It's a reality that's been faced here since time immemorial.

(Manaiakalani Kalua chanting in Hawaiian) (chanting) NARRATOR: Ever since the first people arrived on these bountiful islands, a wealth of beliefs and legends has sprung up to account for these devastating natural disasters.

In Hawaiian culture, Pele, the goddess of fire, resides within the volcano.

(chanting continues) A lot of people have used the term that she's "upset," so she's taking out houses, and that's why there's a lot of destruction and devastation.

She's doing what she does.

We're in her space.

If she takes the house, she takes the house.

If she decides to move around the house and spare the house, she decides to move around and spare the house.

But there's temporary permanence to living within the boundaries of Pele's domain.

NARRATOR: Building a home here has always been a gamble.

And sometimes, your luck runs out.

(Kalua chanting) May 17, two weeks into the eruption.

Just past 4 a.m., cameras on the peak of Kilauea's big sister Mauna Kea catch a huge ash cloud bursting from the summit of Kilauea.

It looks like the summit explosion predicted by the scientists.

News media pick up the story.

Stunning images tonight: that powerful explosion at the summit of Kilauea shooting a plume of ash some 30,000 feet into the air.

NARRATOR: It's the first sign that bigger explosions might be on the way.

But could it get even worse than the 1924 event?

As magma exits the chamber beneath, heading down the mountain, the crater above is collapsing inches per day in a series of earthquakes.

(land and wall shaking) If the collapse continues, it could lead to an even bigger threat.

(explosion echoes) A pyroclastic surge: a superheated ash cloud of vaporized rock and magma capable of flowing across the landscape at hundreds of miles an hour, obliterating all it touches.

Hundreds of years ago, a collapse of Kilauea's crater led to a similar event.

In 1790, there were quite large surges of this hot gas and ash that killed quite a few people.

One of the big concerns was, if this thing gets too big, are we going to go into a pyroclastic phase?

NARRATOR: The local residents are increasingly concerned.

PEEK: Here's another plume coming up right now.

It's maybe 7,000 feet high, maybe eight, I don't know.

But I certainly did not expect to see what I've seen pictures of about 1924, and I'm certainly hoping not to experience what I'd read about from 1790.

NARRATOR: The collapse is a worrying sign that events may be escalating.

Could it be linked to violent new activity now occurring down the mountain?

Around Leilani Estates, the lull is over, but the focus of eruptive activity has moved east.

Two new fissures have opened.

One of them, fissure 17, is pumping out exploding lava.

(explosion echoes) Shooting huge chunks, some as heavy as a refrigerator, hundreds of feet in the air.

These lava bombs are responsible for the eruption's first casualty.

The media report on a resident struck by a bowling ball-sized lava bomb that nearly sheared his leg in half.

The most forceful impact I've ever had in my body in my life.

NARRATOR: So is this a sign that a new type of explosive magma is building up under Kilauea?

♪ ♪ Cheryl's lab quickly analyzes fissure 17 lava.

They find it has an unusual consistency.

Most Hawaiian lava is as runny as ketchup.

It rarely holds on to much magmatic gas.

The gas bubbles in Hawaiian magma can rise freely, and they just basically, just burst at the surface and release the gas.

(explosion echoes) NARRATOR: But fissure 17 lava is rich in silica, the main ingredient in common sand.

The silica thickens the lava to the consistency of peanut butter.

The stiffer your magma, the more viscous it is.

So when gas bubbles are trying to escape, they can't expand very well, and they tend to build up a lot of pressure and then explode.

(explosion echoes) NARRATOR: Most volcanoes around the world are fed by silica-rich magma, explaining the prevalence of explosive eruptions.

If magma this thick were to build up beneath Kilauea's summit, a major explosion could be in the cards.

But Cheryl's team have found chemical evidence that the fissure 17 lava is not fresh from the summit at all.

It seems to come from another hidden remnant of extremely old magma from a previous eruption, even older than the 1955-type magma originally encountered under Leilani Estates.

The chemistry was extremely evolved, meaning that it had been stored longer than anything we'd ever seen erupt on Kilauea.

(explosion echoes) NARRATOR: The longer a magma is stored, chemical changes increase the concentration of silica.

Fissure 17's lava is explosive, but it's tied to a small, isolated pocket of thickened magma.

This discovery indicates that the explosive lava is unrelated to the events at the summit.

And for now, the collapse of the crater is not great enough to trigger a pyroclastic eruption.

(ground shaking) But the ground continues to shake, and the summit is still subsiding as more and more magma exits the chamber beneath, heading for the open fissures.

Down the mountain, the thick silica-rich lava of fissure 17 is starting to congeal.

But as pressure from the summit continues, forceful new fissures are opening behind it, generating new flows heading in many directions.

♪ ♪ NEAL: Understanding where the lava was going was absolutely critical to providing situational awareness for the civil authorities and issuing public warnings in time.

With multiple fissures erupting, this was a challenge.

NARRATOR: Luckily, the authorities can call on some state-of-the-art technology.

We were requested to help participate by Hawai'i County Civil Defense.

And so we said, "Of course, whatever we can do."

(drone buzzing) NARRATOR: Ryan Perroy's team from the University of Hawai'i has been experimenting with drones to track lava flows.

Now their expertise will prove vital.

♪ ♪ By returning repeatedly to a GPS-determined fixed point above the lava's edge, the drones accurately plot precisely how fast a flow is advancing.

And unlike helicopters, drones can safely operate day and night.

(buzzing) ♪ ♪ PERROY: It's critically important to be able to monitor the flows on a 24-hour basis, because even if everybody's asleep, you know, the lava's going to potentially advance.

NARRATOR: Using thermal cameras, they can register the heat signatures of hidden subterranean cracks, helping to forewarn which ones may evolve into new fissures.

And they can help determine which parts of a flow may lead the next advance.

PERROY: We've got areas down here which have cooled off.

These are probably not the places where you're going to expect to have new breakouts.

You're going to see them here and here.

NARRATOR: Drone predictions become vital to the emergency services, who are able to clear everyone safely out of the paths of the multiple advancing flows.

But though tracking its short-term movements can be complicated, if it keeps flowing, a red-hot lava river will always head downhill.

Over three days, the new flows make their way down to the ocean.

But even as they arrive, their power wanes.

The pressure of magma coming down the mountain is now reinvigorating old fissures, moving the center of activity back to Leilani Estates.

(churning) Lava is gushing faster than ever before, and pooling within the hardened banks created by the earlier flows.

Levels are rising by the hour, as lava pumps out at over 40 cubic feet per second.

NEAL: Those rates of eruptive activity, rates of lava flux, really had not been seen at Kilauea before.

PERROY: If it was able to sort of overflow those banks, then all bets are off in terms of where it's going to cover next.

NARRATOR: Residents who thought they'd escaped the worst are about to be hit again.

(lava churning) It was just building up and building up till surface tension gave and it spilled over.

(eruption roaring) MAN: This is insane.

HANSON: Lava's moving, encroaching at the rate of one property a minute.

Very quickly, it became of a scale and magnitude greater than what we were able to handle.

♪ ♪ NARRATOR: The authorities scramble to warn nearby residents, while drones keep an eye on the advancing lava.

(drone buzzing) Watching live drone footage at Civil Defense Headquarters, Bill Hanson suddenly spots something in the depths of the forest.

HANSON: The lights were flashing.

And I looked at it, and I said, "Guys, do you know what he's doing?

He's flashing S.O.S."

NARRATOR: Forced to abandon his car, a resident has become disoriented, his cell phone his only link to his rescuers.

A 911 operator patches him in to the Civil Defense team, who summon the drone to his aid.

HANSON: And so we told him, "Can you see the drone?"

He goes, "Yes, I can see the drone."

And we said, "Please follow the drone out-- the drone will lead you to safety."

NARRATOR: Assisted by the flying beacon, the man is able to find his way out of the forest, and make contact with the rescue team.

And everybody was, of course, elated that we were able to work technology into saving a life.

That was a really special story that we were proud to be part of.

NARRATOR: The eruption has now entered a new phase.

From this point on, one fissure starts to dominate-- fissure eight.

Its flow rate is so great that the spatter forms an immense cone... (lava churning) ...as it pumps out over 3,000 cubic feet of lava per second... (lava flowing) ...forming a vast river hundreds of yards across.

♪ ♪ PERROY: It is an awe-inspiring thing to watch these flows and watch them evolve and change over time, and to see these immense channels and rivers of lava just passing underneath you.

It is a very humbling experience.

♪ ♪ NARRATOR: The new flow powers forward for miles and miles.

It's now clear the effects of this eruption will be widespread and long-lasting.

Countless more acres will be poisoned or buried under a red-hot blanket.

But for the Hawaiian ecosystem, this is not the calamity that it first appears.

The wave of destruction may ultimately generate the seeds of life.

Rick Hazlett, from the University of Hawai'i, is investigating a cave hidden in the rainforest.

A gateway to the Hawaiian underworld, revealing subterranean secrets about the island's formation.

HAZLETT: To enter one of these caves is like seeing the inside of an eruption.

♪ ♪ This is a lava tube.

NARRATOR: Countless numbers of these huge subterranean tubes crisscross Hawai'i.

Some, like this one, can be over 30 miles long.

They are the remains of lava flows that hardened on the outside, but stayed liquid on the inside, continuing to flow for months or years until they ran out of lava.

HAZLETT: So like a dam or a reservoir draining, the lava emptied this passageway and exposed the cave that we can enjoy and explore today.

NARRATOR: You don't see lava tubes in many places around the world.

Unusually hot lava is needed to maintain tunnels that can extend such great distances.

(lava churning) But Hawaiian volcanoes cook up some of the hottest lava on Earth, over 200 degrees hotter than most other volcanoes.

So Hawaiian flows can spread the lava far and wide.

HAZLETT: They're landscape builders.

They're like a metro system delivering passengers here and there.

In this case, the delivery is molten lava, and the product is a new landscape, a larger island.

NARRATOR: Lava flows aren't a curse upon Hawai'i.

They are Hawai'i.

HAZLETT: Essentially, 100% of the landscape that you view in Hawai'i is made up of lava flows piled one on top of the other to build these volcanoes in the sea.

NARRATOR: And it turns out that this hardened lava is rich in nutrient chemicals.

As the lava breaks down, it produces some of the most fertile soils on the planet.

From the ashes of destruction, tomorrow's forests will be born.

And there's another distinctive feature of Hawai'i's volcanoes-- their sheer size.

Take away the ocean, and they are revealed to be the highest mountains on Earth.

At over 30,000 feet, Kilauea's neighbors, Mauna Kea and Mauna Loa, put even Everest in the shade, all built by a powerful and persistent supply of magma.

Hawaiian volcanoes are maybe the most productive volcanoes on Earth.

They erupt very frequently, and just keep piling on more and more and more.

NARRATOR: So what is so special about Hawai'i that it can supply such an abundance of extra-hot magma to its volcanoes?

Kilauea is situated on the southeast end of the island chain.

As you move northwest, the volcanoes become less active, and the lava flows more ancient.

It's a sign that the Hawaiian chain may be formed by a powerful force from the depths-- a subterranean hotspot.

Hotspots are generated by plumes of hot, softened rock, rising up from deep in the mantle.

When they contact the Earth's crustal plate, the magma infiltrates the solid rock.

Over millions of years, as the plate passes over the hotspot, a chain of islands is created.

The plume hypothesis can explain why Hawai'i's lava is so hot and so abundant.

And why these islands grow at such an impressive rate.

In 2018, this growth is on stark display.

In the first week of June, the main flow from fissure eight advances through residential areas.

Hundreds more islanders are forced to evacuate as it makes its way to the ocean.

This time, when it hits, the flow shows no sign of dying down.

♪ ♪ Wherever the front of the flow meets the ocean, there is an explosive reaction.

(explosion echoing) Sending lava bombs hundreds of feet into the air.

(explosion echoing) On July 16, a boatload of tourists get more than they bargained for when they get too close to an explosive front line.

(people screaming) NARRATOR: They are lucky to survive relatively unscathed-- a few nasty burns and one broken leg.

From the violent clash between ocean and lava, new land is born.

Coastal waters have been pushed away, replaced by a lava delta with an area equivalent to nearly 700 football fields.

As long as the volcanoes stay active, this island will continue to grow.

But while volcanic eruptions are busy building up Hawai'i, other processes are busy bringing her down.

Her first enemy is the tropical climate.

(thunder claps) (rain pelting) ♪ ♪ Hawai'i is the wettest state in the U.S.A., drenched by an average of over five feet of rain each year.

This deluge results from the combination of high mountains and an enormous ocean.

Moist air coming off the sea hits the land, rising, cooling, and releasing downpours that begin a process of quiet destruction.

♪ ♪ Volcanic basalt is a porous rock highly susceptible to the elements.

HAZLETT: Rock such as this, the basalt that erupts and builds these volcanoes in Hawai'i, is especially prone to weathering in Earth's atmosphere.

Compared to other rock types, like granite, rocks like this, exposed to moisture for very long periods of time in this warm climate, will slowly dissolve.

You have to think of the Hawaiian Islands as being temporary geographical features.

NARRATOR: The islands are fragile.

Weathering, erosion, and massive landslides mean they are continuously crumbling away.

But more importantly, as the islands drift away from the hotspot, the cooling plate descends, drawing them slowly beneath the ocean.

It's been happening for millions of years.

To the northwest lies the Emperor Seamount chain, stretching thousands of miles-- relics of previous generations of Hawaiian islands, now submerged shadows of their former glory.

In 2018, another island remnant disappeared beneath the waves, the last sands of East Island swallowed by the sea.

So is the Hawaiian archipelago doomed to disappear?

There's new activity just to the southeast of the Big Island.

It's attracted SS Nautilus, a research vessel.

From here, oceanographers are guiding a submersible down into the depths.

DARLENE LIM: We're working down about a mile underwater.

It's very dark there-- it's perpetually dark there.

NARRATOR: The submersible has reached its target.

It's time to light up the scene.

LIM: The steepness of it is so remarkable... CHRIS GERMAN: Look how that drops away down the other side.

LIM: It's incredible.

GERMAN: It's brutal.

NARRATOR: The team find what they are looking for-- a hydrothermal vent.

LIM: What's very exciting is that right now, it is active, and there is hot water that's venting from certain regions.

NARRATOR: The water around the vent is close to boiling point.

It's hot because this piece of land is the summit of Lo'ihi, an active volcano submerged beneath the ocean, created by the same geological hotspot driving the eruptions on the island above.

Lo'ihi is over two miles high, and growing inches per year.

LIM: This is something which will progress on its own time.

We're going to have to wait hundreds of thousands of years, would be my guess, or at least tens of thousands of years, before we see anything that'll breach the surface.

NARRATOR: Lo'ihi will likely form the next island in the Hawaiian archipelago... ...proof that as old islands sink beneath the sea, new islands will rise to replace them.

Hawai'i will survive.

It's weeks later, early October, and Leilani Estates is quiet.

The 100-foot cone of fissure eight, the center of so much activity, lies dormant.

For over a month, the scientists have detected little eruptive activity anywhere on the volcano.

FRANK TRUSDELL: There is no molten lava in this area at the surface.

There is no molten lava anywhere on Kilauea volcano.

For the Leilani eruption, it's over.

NARRATOR: This eruption lasted over three months, producing enough lava to bury the whole of Manhattan nearly 30 feet deep.

It covered nearly 14 square miles, and built 875 acres of new land in the ocean.

♪ ♪ At the top of the mountain, the dreaded summit eruption never materialized.

But the draining of the magma chamber led to an epic slow-motion collapse.

The crater is now 15 times larger, and deeper than the Empire State Building is high.

Those who chose to live on the slopes of Kilauea are counting the cost of their high-stakes bargain with paradise.

Over 700 properties were swallowed by the lava.

Stacy Welch is one of the lucky ones.

Her house is still standing.

WELCH: I'm still in shock.

It's just the power of nature-- it's unbelievable.

That's incredible to me, to have something just come and wipe everything out when there were 700 homes there, and now there's nothing.

There are no words sometimes.

The healing from this is, it's going to be lifelong.

It makes all your other problems look really small when you're dealing with lava.

I would not have wanted to miss it, but I don't want it to happen again.

KALUA: You know, we all know that there's a possibility that we may not have a house tomorrow.

But if we can live with her, if we do what we need to do in taking care of our land, that's the ideal of what I think is being Hawaiian.

♪ ♪ NARRATOR: In 2018, Kilauea proved her destructive potential.

But if science tells us anything, she isn't done yet.

Scientists are confident that someday, in the not-too-distant future, the lava will return.

NEAL: Even though there may not be lava moving across the surface, HVO is still on guard and being vigilant and watching the activity, looking for changes that might mean that more dangerous activity is coming.

This is a very active volcano, and it will erupt again.

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