Original subtitles

NARRATOR: Japan's volcanoes have produced some of the largest eruptions

our Earth has ever seen.

Those volcanoes remain active today.

TOM: It is right now in a very violent phase of activity.

It has been building up, and it continues to build up.

So we don't know what's happening next.

NARRATOR: Could a series of earthquakes be setting off

the country's volcanoes?

TOM: There's an explosion!

Another one, yeah!

NARRATOR: And could lead to a massive eruption.

[Explosion]

Nature's power is our planet's most spectacular show.

♪♪♪

♪♪♪

♪♪♪

♪♪♪

♪♪♪

♪♪♪

♪♪♪

♪♪♪

NARRATOR: Its beauty makes it an iconic Japanese landmark...

but its near-perfect symmetry belies its destructive nature.

Mt. Fuji is a stratovolcano.

Its last major eruption was 300 years ago,

but it is still active, and one day will erupt again.

It's a very appropriate symbol for this island nation.

Japan is prone to violent earthquakes,

and it has more than 100 active volcanoes.

The country is spread over three major islands.

Kyushu in the south is currently the most active.

It has several massive volcanoes,

including two that are considered to be among

the world's top five most dangerous:

Aso and Sakurajima.

Both are serious threats to the island's population

of 13 million.

This is Sakurajima.

It's located on the southern tip of Kyushu.

Scientists pay close attention to this volcano,

and for good reason.

Sakurajima has produced a number of cataclysmic eruptions

in the past few centuries.

The last was in 1914.

What's especially concerning is that it sits across

a narrow strait from the city of Kagoshima

and its 600,000 inhabitants.

A major eruption would put the city and its people

in serious danger.

Volcanologist and photographer Dr. Tom Pfeiffer

has come to Sakurajima just as it's entering

a new phase of larger eruptions.

His timing is excellent.

Just as he's driving into Kagoshima,

Sakurajima erupts.

[Explosion]

♪♪♪

♪♪♪

TOM: We're east of Sakurajima,

which had some massive eruptions earlier today.

The whole northern and western sector is hazy.

That's actually the ash plumes.

Reports say the ash went up to 17,000 feet.

That is more than 5 kilometers altitude

and it's a record number, I think, for this year.

NARRATOR: Tom pulls over to get a better look at this eruption

which is sending a big ash cloud into the sky.

TOM: And it's continuing to spew ash.

And you see all this haze, that's all ash

which is still dissipating from the large plumes

emitted earlier.

Apparently it has been-- it is right now

in a very violent phase of activity.

If you look over the past months

it has been building up gradually.

It continues to build up, apparently.

So we don't know what's happening next.

[Explosion]

NARRATOR: Sakurajima has been in a phase of

nearly continuous eruption since 1955.

It normally erupts once or twice a day.

But in the past few weeks,

the frequency of the eruptions has increased exponentially:

as many as 10-15 times a day,

with ash plumes up to five times larger than usual.

Scientists still can't predict the timing of a major eruption.

But they are afraid that this volcano is building up

to something major.

An eruption similar in magnitude to 1914 could be devastating.

It could potentially destroy the city of Kagoshima.

TOM: From here you get the sense of how close Kagoshima City

is actually to the volcano.

Volcanologists know that the volcano has been inflating

gradually ever since the 1914 eruption,

and it has reached the same level as it had

before the last big eruption.

NARRATOR: What that means is that the volcano

has replenished itself enough that its magma reservoirs

can produce a major eruption,

something it has done several times

over the past 600 years.

TOM: So, it is thought that it might be actually due

at some point, not necessarily tomorrow or the next month,

but in the not too distant future it could be due

for another larger eruption.

That would clearly put the city at risk--

particularly of pyroclastic flows,

if you have an eruption column that rises,

imagine 15-20 kilometers.

NARRATOR: That could be catastrophic for the city.

Large eruption plumes can collapse under their own weight,

causing a burning hot avalanche of ash and gas

called a pyroclastic flow.

They are capable of generating tsunamis

or even traveling over the water

on a super-heated layer of steam,

burning everything in their path.

TOM: Those pyroclastic flows could sweep and engulf the whole city.

To evacuate a large city like Kagoshima

is not an easy task, clearly.

If you look at the lower slopes you see this dust floating.

This is ash from the recent eruptions

that has been blown around the island.

And people who live there have that constantly.

Anything you wash, after a day, is coated with ash.

NARRATOR: Sakurajima means 'cherry blossom island.'

It was given that fanciful name because its explosions cover

the volcano in glowing rocks that resemble a tree in blossom.

Even though Sakurajima is very active,

travel to the island on which it sits is still possible.

After Tom takes the ferry over, he'll be able to drive

to the active Showa crater, which is on the far side.

But there are hazards.

Getting there means he'll have to pass right through

the ash cloud.

TOM: This is clearly where the plume had passed

that was produced by these big explosions earlier today.

And in fact you can see a lot of ash covering the leaves.

Look at the car in front of us and, you know,

this ash is everywhere.

This can be very heavy if you have to live here

and this is something you have every few days.

But people go on living here and they stick to their place.

They don't want to give this up.

NARRATOR: This tourist shop is as close to the volcano

as Tom can get and still be safe.

The area beyond it is off-limits.

TOM: What we're seeing is that the eruption is continuing.

With every few seconds you see jets of material

and lightning now in the ash plume rising.

And they contribute to this sustained plume

that reaches 1-2 kilometers above the vent, and spreads,

and all that Northeastern sector is under ash fall.

It's quite spectacular, actually.

NARRATOR: This is not typical behavior from Sakurajima,

and it's likely attributed to a new pulse of magma

coming up from depth that has lead

to a more sustained ash eruption.

TOM: Let's hope the wind stays as it is,

because we are outside the ash plume

and we have a clear view to the crater.

This is great.

Let's pray it stays like this.

On the other side, it's miserable now.

You have dust in the air and ash falling,

and you have to be inside, lock the windows.

If you're outside, you'd better wear a face mask.

NARRATOR: Sakurajima's usual eruptions are a single blast

that only lasts a few minutes.

This one has gone on for several hours.

TOM: The question is, could this be a precursor

to a much bigger eruptive phase?

I don't dare to say.

On the long term, maybe.

NARRATOR: Although volcanoes can and do erupt without warning,

most large-scale eruptions have a build up phase

as more magma is pushed into the system.

As this is happening, there are many localized effects,

like earthquakes, that can be detected

by high-tech instruments, or sometimes even felt

by the people living around the volcano.

TOM: Before the 1918 eruptions,

there has been a very marked increase

in earthquake activity.

Visible deformation of the-- inflation of the ground.

The coastline visibly changed,

and things like this are expected to happen

as a precursor of another big eruption,

which would imply a much greater volume to erupt.

By greater, I mean orders of magnitude greater:

a hundred times more than what has happened today.

NARRATOR: Before the 1914 eruption,

the city of Kagoshima was rocked by three weeks

of very strong volcanic earthquakes

that caused a number of fatalities.

The quakes are triggered when new magma breaks the ground rock

as it pushes up to the surface.

TOM: What interests volcanologists to study is actually

to have a picture of what is actually happening underneath.

And understanding what is happening underneath

might eventually help to have an idea

about what is going to happen.

Unfortunately, it is not very well understood,

and it is extremely difficult to be sure and certain

what is happening.

NARRATOR: Scientists don't fully understand

what triggers big eruptions.

The current belief is that once a magma chamber is filled,

as Sakurajima's, any new large injection

of fresh hot magma can reinvigorate

the older semi-cooled magma in the volcano's reservoir.

The heat and pressure of the new material

can trigger a large eruption.

Eruptions are limited to the amount of eruptable magma

within the underground reservoir.

A big event can virtually empty that chamber.

Right now, scientists believe Sakurjima's chamber

has refilled to 90%,

which makes a larger, 1914-size eruption possible.

Or it can erupt moderately like we are seeing here,

gradually reducing the pressure within.

That scenario could mean that a major eruption is less likely

since the volcano can't build up enough pressure.

NARRATOR: The other concern is that these smaller eruptions

can also open the conduit that connects to the magma chamber.

If that happens, the result is like popping the cork

on a champagne bottle.

The eruption can then sustain itself

until the magma chamber is virtually empty.

That scenario could happen quickly

with little very little advance warning.

TOM: Sounds like a thunderstorm in the distance.

NARRATOR: The activity is spectacular, but today, at least,

is not the precursor to a larger eruption.

Things wind down as the night goes on.

But after dark, Tom can see a glow within the crater.

That confirms that the magma is able to push

all the way to the opening of the crater vent.

By morning the ash emission has nearly stopped.

It's not over-- the volcano is recharging

before it begins another eruptive phase.

The cycle that Sakurajima is going through

is considered high, but still in the realm of normal.

In Japan, the news of Sakurajima's increasing activity

is watched closely by an anxious population.

What has touched off public fear is that several volcanoes

have become active at the same time--

volcanoes that have been quiet for decades or centuries.

They're the latest reminders of just how geologically active

the country is.

In 2011, Japan was rocked by a major earthquake and tsunami

and the subsequent meltdown at the Fukushima nuclear plant.

More than 19,000 people died,

and the entire area was evacuated.

The Tahoku quake was a national trauma

that has increased awareness and raised alarm

about the potential of more natural disasters.

♪♪♪

♪♪♪

NARRATOR: And in 2014, 50 people were killed

when Mount Ontake erupted suddenly and without warning.

The eruption sent a pyroclastic flow

down the mountain, killing unsuspecting hikers.

On top of that, Hakone, the volcano closest to Tokyo,

has been raised to alert level 3 out of 5

after thousands of small earthquakes

and a sharp increase in heat was recorded at the summit.

There are fears that it too may soon erupt.

The Japanese media gives these stories prominent coverage.

The population wants to know the government

and its civil defense force have plans in place

in case of another disaster.

While Tom is exploring Sakurajima,

a major eruption happens several hours south.

Mount Shindake, on the Island of Kuchinoarbujima,

erupts violently, sending a huge ash cloud into the sky

and triggering a potentially dangerous pyroclastic flow.

The eruption was caught on a government web camera

and it shows the sudden horrifying nature

of the explosion.

This powerful eruption could have had dire consequences,

but luckily the island had few inhabitants.

There were only two injuries.

♪♪♪

♪♪♪

NARRATOR: Eruptions of this magnitude are infrequent,

so Tom has left Sakurajima

and is traveling by ferry to Yakashima,

the island directly across from the eruption.

If conditions are right,

it will give him an excellent place

to see the aftermath of the eruption

and to see how Japan responds to a volcanic crisis.

Japanese media has descended on the local police station

that has become the command center for the disaster.

[Reporter speaking Japanese]

TOM: Yesterday about this time, a very large explosion occurred

on the neighboring island of Kuchinoarbujima.

It was an incredible, large, unexpected explosion

that forced the evacuation of the whole island.

We see lots of TV crews.

They're all covering this story,

which is the headline all over Japan.

I bet nobody expected it to be that large.

It produced an eruption column 9 kilometers high,

which spread out, and the base of the column

part of the ejected material immediately collapsed

and caused a violent pyroclastic flow

which fortunately didn't go through exactly

the populated area, it seems.

But it was very close.

It illustrates a bit the problem,

if you live on an active volcanic island,

in case of a large eruption like the one yesterday,

there are not many places you can go and hide.

NARRATOR: Tom heads to the lighthouse at Nagata Cape.

It's the closest point to Kuchinoarbujima Island

and is the best vantage point to see the volcano.

But bad weather and the heavy concentration

of airborne ash from the eruption

have reduced visibility considerably.

Tom can't see the volcano.

TOM: Unfortunately, the island is, of course,

and it makes sense, off-limits.

And the weather is not cooperating.

We can't even see it.

But we gave it a try.

In case there would be another eruption

you should be able to see the plume,

and maybe in clear weather you can even see the island.

It's already quite a bit of luck

to be in the vicinity of this event.

NARRATOR: The media is focusing on questioning if these volcanoes

are becoming more active because the of the Tahoku quake.

TOM: It has always been questioned,

and many people tend to think

that there must be a correlation between

earthquake activity and volcanic activity.

And in fact, there is probably some relation

but it's currently not even well understood.

NARRATOR: To a volcanologist, this eruption is another chance

to study the phenomena up close.

But for a nation still shaken by the earthquake

and tsunami at Tohuko, this large eruption

once again raises collective fears.

Japan is a very geologically active country.

On a relatively small landmass,

it has more than 100 active volcanoes.

Japan's abundant earthquakes and volcanoes

can be explained by the science of plate tectonics.

The Earth's surface is broken up into

just over a dozen large sections or plates.

They are constantly moving,

at about the rate a fingernail grows.

When two plates collide, the denser of the two

is forced under the other and back into the earth.

The place where they meet is called a subduction zone.

As the denser plate descends, friction causes stress to build

to the breaking point.

It slips suddenly, causing an earthquake,

and sometimes a tsunami.

As the denser subducting plate is forced downwards

into the hot mantle the water and sediments

that are pulled down with it begin to boil off.

This lowers the melting point,

and the rock around begins to melt, creating magma.

The more buoyant magma rises towards the surface.

Japan is the result of a major subduction zone.

TOM: You have to consider that it is only roughly 50 years

that the whole concept of plate tectonics

has been accepted,

and it is still a science really in evolution.

NARRATOR: For now, it's a theory that scientists can test against.

TOM: Right now, of course, each major earthquake,

each volcanic eruption provides a new opportunity.

You can test your previous models.

You have a fresh case.

As science advances and monitoring techniques--

instruments become more refined, you have the ability

to measure and detect more subtle changes

that you would not be able to see maybe five years ago,

as technology constantly improves.

But it's not very easy to establish a rule to say

"We have a big earthquake,

so for sure we will have volcanic eruptions."

NARRATOR: At this point it is unlikely Mt. Shindake will erupt again.

So Tom spends time observing how the emergency response team

is handling the situation.

Japan has been sharpening its skills

when it comes to disaster response.

Within hours of the eruption,

the island was completely evacuated.

By the end of the day mobile command centers had been set up

around the island to relay information

back to their operation headquarters.

Tom is with a just a few members of the

Kagashima Civil Protection Communications division.

This is the same team that would be responsible

for an eruption of Sakurajima.

TOM: This island is approximately 150 kilometers away,

and it's in a quite remote area,

so they're doing an amazing job.

They're showing us the camera that they had

on the helicopter who arrived on the scene

just 40 minutes after the eruption.

Then the main team followed right afterwards

and is now based here.

And yeah, they are here, and it shows--

it's quite impressive to see the level of alertness

of the Japanese authorities

in response to a volcanic crisis.

NARRATOR: The teams are equipped to relay video and data

in real-time back to the command center in Kagoshima.

All of this can be packed and loaded on a helicopter

and be on site within an hour.

Their vehicles are equipped with generators

capable of powering a small village.

TOM: In this case, it was a small island

with approximately 150 inhabitants.

Now you have to of course try to imagine what you would do

in the case of an eruption at Sakurajima of that scale,

where you have 700,000 people living in the city.

NARRATOR: The eruption was powerful.

It will likely be rated a VEI-3

on the Volcanic Explosivity Index.

The Civil Protection Force will remain in place

for several weeks in case there is a secondary eruption.

The response to this explosion was swift and effective.

No lives were lost.

It's another chance for this unit to hone its technique,

because in Japan the question isn't

will there be another eruption, but when.

TOM: I think if any country is really preparing

for worst-case scenarios, a large explosion of Sakurajima,

Japan is probably the best prepared on a global scale.

NARRATOR: Tom heads back to Kagoshima to explore a sleeping giant

so massive that its potential threat

is almost impossible to comprehend.

This is the explosive center of an ancient volcano,

so massive that this entire area,

including part of the city of Kagoshima and Sakurajima,

are sitting above a giant magma chamber.

Deep beneath the waters of Kagoshima Bay

is a much bigger menace...

the giant Aira Caldera.

TOM: The entire city literally sits inside a volcano.

You can see the cliffs over there.

It goes all around,

from about 24 kilometers by 18 kilometers,

a rectangular area, that is the caldera.

It's probably the worst place for a city to be.

NARRATOR: A caldera is a feature that happens

after an eruption empties the magma chamber

and the ground above collapses into it.

The eruption of Aira Caldera took place 22,000 years ago,

and in the years since,

there hasn't been a bigger eruption on Earth.

Its effects can be found all around the bay.

TOM: This hill, "Shiroyama," means "white mountain," literally.

Shiro, white; yama, mountain.

White, you can see why; it's full of pumice.

The white pieces of pumice and fine ash.

It's the same material as pumice,

and darker fragments.

Geologists call this a non-welded ignimbrite.

It's the result of a massive-- a giant volcanic eruption.

This is just a tiny part of the deposit.

The whole cliff is probably 100-200 meters high.

It's composed of this material.

And in fact, this eruption was so large

it deposited pumice and ash all over Japan.

NARRATOR: The eruption was so massive it was believed to be

200 times the scale of the 1980 eruption

of Mt. St. Helens in the United States.

Mt. St. Helens sent a huge ash cloud into the air

so big that deposits were found in 11 states

and five Canadian provinces.

It was rated as a VEI-5 eruption

and one of the largest in recent times.

Eruptions are measured using the VEI

or Volcanic Explosivity Index.

They're based on the amount of material ejected

and the height of the ash plume.

The scale ranges from 1 to 8.

VEI-1 is gentle.

VEI-8 is mega-colossal.

Each successive number is 10 times greater in scale

than the previous.

An eruption is classified from start to stop.

The current eruption phase of Sakurajima

has lasted more than 70 years

for a total combined rating of of VEI-3.

Compare this to Sakurajima's 1914 eruption,

which lasted just eight months, was rated VEI-4.

Aira's caldera eruption ranks as super-colossal,

between VEI-7 and VEI-8.

An eruption of this magnitude

hasn't happened in recorded history.

The closest is the 1812 eruption of Tambora in Indonesia,

which was a VEI-7.

That eruption killed 92,000 people

and caused widespread crop failures throughout Europe

that lasted several years.

The Aira eruption 22,000 years ago

was eight times more massive than that.

TOM: This massive giant eruption

or if you want to call it a super-volcano eruption,

a super eruption, had a result

that the crust of the Earth sank several hundred meters

forming what is called the "Aira Caldera,"

and then a large 24x18 kilometer caldera formed.

This whole city actually sits inside the Aira volcano.

Aira volcano is the whole complex, the whole caldera.

We are here on the rim;

we are looking down onto the city.

This is the crater floor.

NARRATOR: A super eruption is fed by the build-up

of a lot of magma in the volcano's reservoir.

Plate tectonics explains how that works.

Instead of two plates colliding,

where one sub-ducts under the other,

around Japan several smaller micro plates are being crushed,

twisted, and pulled.

This movement has made the Earth's crust very thin

under the island of Kyushu.

To make matters worse, the direction and speed

of the sub-ducting plate has created a convection current

that is producing an anomaly called a mantle wedge.

This brings superheated magma even closer

to the surface, where it reacts with ground water

to lower the melting point of the overlying rock layers.

It's the volcanic equivalent of the perfect storm.

The higher temperatures of the mantle material allows for

a lot more magma to be produced at shallower depths.

It creates conditions for massive eruptions.

This is what's happening under Japan's Kyushu Island...

and why the island is littered with active

and very dangerous volcanoes, including Aira.

Although Sakurajima appears to be a volcano

within a volcano, they are one and the same.

Sakurajima is a vent of Aira, but has its own magma reservoir,

but that reservoir draws its magma from Aira's.

That is one of the reasons it can produce large

VEI-4 to 5 eruptions every 100-200 years.

TOM: The whole island of Sakurajima is actually

two overlapping stratovolcanoes,

An older one, Kita-dake, to the left,

and the young one, presently active Minami-dake.

All this has grown just within the past 25,000 years,

so this is really an incredibly active volcano.

It is one out of the ten most active in the earth,

I would say, at the moment.

It is definitely Japan's most active one.

NARRATOR: Scientists still don't fully understand what triggers

a caldera eruption.

This is where the massive reservoir erupts.

During Sakurajima's 1914 eruption the entire caldera

dropped by almost two feet lengths

proving that the two magma chambers are connected.

But the big and unanswerable question is,

is that enough to destabilize Aira Caldera?

For the people of Kagoshima, the good news is

that super eruptions are infrequent.

Such a massive blast requires long periods

between eruptions to rebuild the intense volume

of molten material within the magma chamber.

That process takes many thousands of years.

Over the past 300,000 years there have been several

massive caldera eruptions on the island of Kyushu

at 20,000-40,000 year intervals.

The volcano that is most likely to produce a caldera eruption,

and in a more advanced stage of recharging,

is Aso Caldera.

Aso Caldera is not just bigger than Aira's,

it's one of the biggest on the planet.

It is ranked as one of the most dangerous volcanoes

in the world.

TOM: This caldera belongs to the XL class of calderas.

There are not that many in the whole planet

that have been active

in the past few hundreds of thousands of years.

Maybe you can count them in terms of dozens.

NARRATOR: Aso sits directly over the apex of the mantle wedge.

Here the crust is also the thinnest in Kyushu

and Aso has produced four large caldera eruptions.

Three of them happened within 30,000 years of each other.

On a geological timescale, that is very close together.

The last one was 90,000 years ago.

Aso also contains a geological feature

called a resurgent dome.

This is where inflation from the magma chamber

pushes the caldera floor upwards,

indicating a charged magma chamber.

It's estimated that the volume of new magma

coming into Aso's reservoirs is so great

that it is capable of producing a VEI-7 eruption

every 10,000 years.

It has been recharging for 90,000 years,

so it's capable of something massive any time.

But over the past 20,000 years

its activity has been relatively small,

and the impact of its eruptions has been localized.

There are five major peaks and 17 cones.

One of them, Mt. Nakadake is still active.

TOM: This bluish haze, it's very typical

if there's SO2 gas present.

That is very typical if magma is actually

very close to the vent.

So I suspect the eruption is still going on.

It is just right now maybe a bit quiet,

but it could produce explosions any time.

Of course, if a sudden explosion occurs

it's extremely hazardous,

and in fact, people in the past

unfortunately have been killed by projectiles.

NARRATOR: The SO2, or sulfur dioxide gas, is toxic and an obvious danger.

In fact, Aso has a history of small but deadly incidents.

Two people were killed in 1979 from a sudden explosion

that caught everyone off-guard.

Eleven others were injured when a rock crashed through

the roof of a bombproof shelter.

Tourists had been able to take a cable car

to the crater,

but that was suspended just over two years ago,

when the volcano alert level was raised to 3.

Tom is going to get a rare view.

He has arranged special permission

with government officials to visit the active crater.

The current eruptive crater of Nakadake

has seven vents that have erupted in historical times.

You can tell how active a volcano is

by how quickly it builds new mass.

TOM: Very large, extremely active volcano,

Remember, all these mountains with all these cones,

it's just the central part of the much larger complex,

and it's only 90,000 years old,

which is in geologic terms almost nothing.

So for a volcano it's still very young,

a very young age.

Wow, yeah... It's a very impressive crater.

You have no idea when you're down there;

it looks so flat and harmless.

But this is actually something.

Wow, yeah, incredible.

Wow.

Okay, we are there.

It's impressive.

The Nakadake crater is all this, actually.

It goes on over there.

It is more than a kilometer long in total.

It has seven different vents,

out of which we see two here that are active.

You hear the roaring,

so there is a very intense degassing going on.

Magma is very close to the surface.

Explosions could actually occur any time.

Number 2 crater is also active.

There's a lot of degassing going on.

It's not so much steam, but apparently lots of SO2.

This bluish haze is created by SO2.

We're almost out of the plume,

but you still can smell sulfur now.

Wind is swirling a bit.

We're at the edge of this haze.

So I guess if we were right in the plume

we'd probably need the gas mask to breathe.

NARRATOR: The gases coming off the plume are deadly.

In 1997, three people were killed on this spot

when they were overcome by the noxious gas.

TOM: Nobody can predict whether it will just gradually

calm down now in the coming weeks and months,

or it might re-intensify.

A couple of months ago you could almost see

constant Strombolian activity.

At least from the webcam. I wasn't here, of course.

So you would have seen incandescent pieces of lava

being ejected, and they piled up to form that cone.

NARRATOR: Up until recently, the crater contained a lake.

But the water boiled off because of the extreme heat.

Water from the surrounding hill drains into the crater,

making this volcano unpredictable.

TOM: I find this is extremely impressive.

The contrast couldn't be...

There's an explosion!

Explosion!

This is a phreato, maybe phreatomagmatic explosion.

You see the ash?

NARRATOR: The eruption is a result of ground water

coming into contact with hot magma.

It flashes to steam.

As a gas it expands to more than 1,200 times its volume

in a sudden explosion.

Much of the plume is water vapor.

TOM: You could probably see stones being ejected.

This was a very small one,

but well, that was fantastic!

Another one, yeah.

TOM: You see the black fingers of ash,

followed by white steam.

This is very typical for phreatomagmatic explosions.

[LOUD EXPLOSIONS]

TOM: It's easy to be killed here.

If this volcano was not monitored,

if there was free access

you would have hundreds of people maybe

on a good day, walking around here.

And then, of course, the more people there are

on the rim, the more likely it is

that during an explosion there could be an incident.

And during daylight especially they're not glowing,

you have very little chance to escape them.

Even the hard hat would not--

it gives protection against small things,

but if you have a block like this the hat is useless.

Imagine this 100 times bigger, and we'd be in trouble.

For me this was absolutely amazing.

NARRATOR: Although the eruption was small and not threatening,

the situation has now become unpredictable.

The officials have asked everyone to leave.

TOM: The best way to decrease risk is to limit the time

in a hazardous place.

So we've seen two really nice small explosions,

so we don't want to risk to get to a bigger one,

which could happen.

So that's legitimate.

They know the volcano, and they don't want to go

and take us to any higher risk levels.

So I completely understand.

NARRATOR: Large caldera volcanoes work on a geological clock,

taking tens of thousands of years

to ready themselves for a major eruption.

And in those terms, Aso either be could be

overdue for a caldera eruption,

or it could still could be thousands of years off.

Given its size and history Aso is capable of producing

a massive eruption.

Everything in its danger zone could be wiped out,

including Kumamoto City, which is downslope

from the volcano.

In recent history, the only eruption we can relate it to

is the 1991 eruption of Mt. Pinatubo

in the Philippines.

It was the largest eruption to affect a populated area

in the 20th century, with a ranking of VEI-6.

It caused incredible devastation to the surrounding area.

The ash fall was recorded in places as far away as Vietnam,

Cambodia, and Malaysia.

That eruption was an important landmark.

It was the first time that scientists were able

to predict the eruption in time to do mass evacuations.

Because of that, the death toll was under 1,000.

But if history repeats itself

Aso would be on a vastly larger scale.

What they'd be looking at would be 40-50 times bigger

than Pinatubo.

This type of eruption can't be planned for or mitigated.

The ash fall alone would be so huge

that it would affect all of Japan.

Its impact would be global.

But hopefully it isn't something humans have to deal with

for thousands of years.

♪♪♪

NARRATOR: Back at Sakurajima, that volcano's explosive activity

has continued to ramp up.

TOM: Five to ten years ago it was maybe an average

of one eruption, two.

Maybe on a good day there were two per day.

There has been a clear increase over the past years, though,

so it's not unusual now to have maybe up to ten

during phases of elevated activity.

NARRATOR: It's likely that Sakurajima

is building to a larger eruption.

And that could mean something in the range

of a VEI-4 or VEI-5 eruption in the not-too-distant future.

TOM: This year alone has produced more explosions

than last year combined, and we're only in May.

Nobody can predict what the volcano is going to do,

but it may be a long-term precursor

to a major phase of activity again.

[Explosion]

♪♪♪

NARRATOR: Tom came to Japan just in time to catch Sakurajima

at its most spectacular period of activity in years.

TOM: Sakurajima has put on a great show for us.

We've seen tremendous explosions,

small and large.

This beautiful bay with fishing boats...

I've never seen Sakurajima so active before.

[WAVES SPLASHING ON BEACH]

♪♪♪

NARRATOR: Japan has no choice but to grapple with giants.

From the sea floor to its tallest peaks,

the country is at the mercy

of the most elemental geological forces.

These forces have created an environment

of staggering beauty,

but they've also caused devastating disasters.

Science is still on a steep learning curve

when it comes volcanoes,

but with each new eruption they are beginning to understand

the incredibly complex forces that are at work

beneath our Earth's surface.

Knowledge that is teaching them how to predict

large eruptions before they happen.

In a country riddled with active volcanoes,

some of which erupt on a daily basis,

life goes on for Japan's nearly 130 million people.

What else can be done in the face of such powerful forces,

except watch and wait...

and hope that we learn what we need to

before our geological time runs out.

♪♪♪

♪♪♪

♪♪♪

♪♪♪

♪♪♪

♪♪♪

♪♪♪

♪♪♪