Original subtitles

(explosion)

NARRATOR: Mount St. Helens-- the biggest volcanic eruption

in North America in nearly a century.

Virtually all life for 200 square miles is wiped out.

It seems impossible that life could ever return

to this barren wasteland.

We found a lot of our conventional wisdom

was just flat wrong.

NARRATOR: In recent years there are ominous signs

the volcano is awakening.

MAN: These things were like skyscrapers that were being

shoved out of the ground.

They were literally that big.

NARRATOR: A 30-year quest to understand

one of the most complicated volcanoes in the world

is revealing new mysteries deep inside the mountain.

MAN: We don't know whether it's going to erupt explosively again

in two years or in 20 years or in 200 years.

NARRATOR: Is Mount St. Helens preparing to erupt again?

Right now, on NOVA--

"Mount St. Helens: Back from the Dead."

Major funding for NOVA is provided by the following...

Supporting NOVA and promotingg public understanding of science.

And the Corporation for Public Broadcasting,

and by PBS viewers like you.

NARRATOR: October 2004.

Mount St. Helens comes back to life.

Steam and ash spew from the crater on the mountain's summit.

We saw the boiling material come out of the ground,

we saw that it was blasting up, it was dark

and it was light at the same time.

It made a plume that rose up over the rim of the caldera.

It came up to above our altitude,

to 10,000 or 12,000 feet.

NARRATOR: It's a frightening development.

(explosion)

For years, Mount St. Helens has been quiet.

The volcano went from quiet to unrest to eruption

very, very rapidly.

NARRATOR: It could be headed for a massive explosion.

DZURISIN: It seemed possible that we were headed

toward an explosive eruption.

We didn't know.

That was a key question.

NARRATOR: The effort to understand what is happening

inside the mountain couldn't be more urgent.

Is the volcano about to repeat the events

of three decades earlier, when it shattered the tranquility

of its peaceful surroundings?

(bird screeches)

NARRATOR: Spring 1980.

Mount St. Helens is one of the major peaks

in the Cascade Mountains.

It's an area of outstanding beauty, rich in wildlife.

For over 120 years, the volcano has been quiet.

But in recent weeks it's been rumbling.

Nobody is sure what to expect.

Then, on May 18, 1980,

a 5.1 magnitude earthquake rocks the mountain.

Within ten seconds,

the volcano's northern flank collapses

in the largest landslide in recorded history.

It releases millions of tons of magma in a colossal explosion.

A cloud of searing gas and rock, known as a pyroclastic flow,

races over the surrounding countryside.

Forests are flattened.

Four miles below the summit,

an enormous lake is choked with debris.

The eruption continues to shoot poisonous steam and ash

miles into the air.

It was just, again, astounding is the best word

to describe what happened in 1980 here in Mount St. Helens.

NARRATOR: The northern slope of the mountain is buried

in several feet of ash.

Virtually all life is extinguished.

57 people are dead.

They include loggers, campers, scientists and a reporter.

Some are up to 13 miles away in areas considered safe.

The plume of steam and ash rises miles into the sky

for the rest of the day.

The drifting ash cloud disrupts air traffic

for hundreds of miles.

The scale of the destruction is enormous.

NARRATOR: Across more than 200 square miles,

the surge of ash and rock incinerates trees.

Thousands of birds

from more than a hundred species disappear.

Billions of insects are gone.

Deer and elk are wiped out.

This vast area of devastation becomes known as the blast

or blow-down zone.

Nearer the crater, ash and rocks from the landslide

litter the northern slope of the mountain.

It looks like the moon.

It's called the pumice plain.

It's directly below the crater.

Four miles from the volcano,

the enormous Spirit Lake is scarcely recognizable.

The avalanche has lifted its bed more than 200 feet.

The surface is smothered in dead trees.

Hundreds of species of aquatic life, including insects,

amphibians and fish, are killed.

CHARLIE CRISAFULLI: It was black water.

And it de-gassed and bubbled,

and there were hot springs that were coming up.

If you were to put your fingers

in to your wrist and wiggle them, you wouldn't even be able

to see your fingertips.

That's how grossly modified the water was.

NARRATOR: Mount St. Helens is now

a lifeless jumble of shattered forest, rock and ash.

It's hard to imagine life will ever return.

The eruption was so powerful,

it altered the shape of the mountain.

Mount St. Helens was a typical cone-shaped volcano

known as a stratovolcano.

But the landslide has torn 1,300 feet off the summit,

leaving a gaping crater a mile wide and 2,000 feet deep.

It's the largest volcanic eruption in North America

in nearly a century.

(helicopter blades whirring)

Weeks after the eruption, scientists arrive at the crater.

The volcano is still steaming and rumbling.

It's a new and unfamiliar world.

One of the first to arrive is Dan Dzurisin.

DZURISIN: There was a tremendous amount of steam

and you could see that it was very hot.

You didn't see red lava oozing out of the ground.

You didn't see fantastic fire fountains.

There was this constant background roar of rocks

cascading down the crater walls.

Occasionally a very large rock the size of the helicopter

would come bouncing down and you could watch it

and it was almost slow motion because the crater was so large.

NARRATOR: Mount St. Helens has a long history of eruptions.

More than 500 years ago, two massive explosions took place

within two years of each other.

They were nearly four times larger than May 1980.

The mountain sits on one of the most active seismic zones

in the world, the Pacific Ring of Fire--

a vast arc of volcanoes running for thousands of miles.

It's home to some of the biggest and most dangerous volcanoes

active today.

Here, the enormous plates making up the earth's crust

are being squeezed together.

Along the coast, the plate below the Pacific

is sliding un the North American plate.

60 miles down pressure and friction melt the rock.

Magma wells up.

When it reaches the surface, it bursts out.

(explosion)

But there are still many unanswered questions.

Scientists' understanding of what triggers an eruption

this massive is incomplete.

And given the scale of destruction,

they need to find a way to predict

when it might happen again before it's too late.

Mount St. Helens is about to become

one of the most intensely studied volcanoes in the world.

The mysteries are not just geological.

Biologists want to know if any life has survived

and what its future will be.

Charlie Crisafulli, one of the leading experts

on the mountain's ecology, arrives soon after the eruption.

CRISAFULLI: Nothing could have prepared me for the sights and sounds

that I saw when I got here.

It was complete and utter barrenness

and there was no sign of life whatsoever.

NARRATOR: His job is to survey the mountain,

looking for any living things.

CRISAFULLI: It was just intriguing to think about how would life come back

to this landscape.

What would the pattern be?

How would the rate be?

NARRATOR: Much of the mountain is still inaccessible.

So he starts work in the blow-down zone

in an area some eight miles downhill from the crater.

CRISAFULLI: We flew over in a helicopter very close to the ground.

We would have these bumping, twisting flights

across the landscape following a contour.

NARRATOR: In the first three months,

there's nothing but dead and uprooted trees.

Then he notices something...

Signs of freshly disturbed earth.

CRISAFULLI: Lo and behold, in many locations,

brown earth on top of the gray volcanic ash.

NARRATOR: Is there something down there?

Crisafulli returns on foot to investigate.

There, emerging from the ash, is a tiny burrowing animal.

It's a northern pocket gopher.

CRISAFULLI: It was very thrilling.

NARRATOR: How can it possibly have survived when nothing else has?

CRISAFULLI: This tiny animal lives entirely beneath the ground.

And so when the blast occurred,

it would have been safely protected

beneath a mantle of soil

and may very well have survived in many locations.

NARRATOR: Over the following months, he finds more gophers.

It appears that life is returning to the mountain

just three months after the eruption.

NARRATOR: By fall, the volcano is still active.

Plumes of steam and ash

continue to shoot thousands of feet into the air.

But something else is happening, too.

The crater floor appears to be moving.

Is the mountain preparing for another major eruption?

DZURISIN: We would sometimes notice a crack

that hadn't been there the day before.

And by the end of the day the crack was larger.

And you realized that the ground was moving beneath your feet.

NARRATOR: It's an unsettling experience

to stand on the floor of a volcano that's visibly moving.

If you stood and looked very, very, very carefully,

with a reference point in the background,

sometimes you could see it move, but just barely.

NARRATOR: The scientists set up a time- lapse camera on a nearby ridge.

Over several days, the pictures show a dome

rising in the middle of the crater floor.

The volcano is oozing a sticky gray lava,

cooling as it reaches the surface.

Over several months, the dome grows larger.

The geologists are puzzled.

What is going on inside the mountain?

Is Mount St. Helens simply rebuilding its summit

or is it about to blow up?

DZURISIN: We didn't know what might come next,

whether the lava might continue to grow for many years--

or even decades or centuries--

and we didn't know if there might be

explosive eruptions in store.

NARRATOR: Then winter closes in,

restricting access to the mountain.

The scientists' work is limited.

Answers will have to wait until spring.

(bird singing)

Spring 1981, nearly a year after the initial eruption.

Life returns to the hills and valleys of the Cascade Range.

But on Mount St. Helens,

the devastation of the previous year is still obvious.

Despite the danger,

Crisafulli moves closer to the active volcano's core.

The pumice plain is buried in several feet of coarse ash.

It's a dusty, barren wilderness.

Life seems impossible.

CRISAFULLI: This is an area where super-hot incandescent flows came down

and killed all life that was here.

NARRATOR: Helicopter is the only way in.

CRISAFULLI: We were flying back and forth, very low,

just above the ground surface, looking for any form of life.

NARRATOR: He crisscrosses the area but there's nothing to see.

Then suddenly, amidst the acres of barren rock,

there's an unexpected flash of color.

CRISAFULLI: So we set the helicopter down and we walked up.

Right out in the center of the pumice plain we saw a plant.

NARRATOR: At first, Crisafulli can hardly believe his eyes.

It was a prairie lupine, a species that typically grows

high in the slopes of Mount St. Helens.

NARRATOR: It's not only growing, it's flourishing.

CRISAFULLI: Not only had the plant established,

but at that point was in full flower.

And it was quite remarkable.

When we saw the first one we were very surprised.

NARRATOR: It's only four miles from the volcano's crater.

It's the first sign of life

in an area where everything has been extinguished.

But how has the plant managed to grow in such a barren area?

The answer is a special root structure

that provides its own fertilizer.

These are little factories where a bacterium works with the plant

and provides nitrogen to the plant.

In return the plant provides the bacterium with simple sugars

that it fixes through photosynthesis.

And so this is a great relationship

where you scratch my back, I'll scratch yours.

NARRATOR: This special process means lupines can grow

in even the most inhospitable terrain.

The lupine, like the gopher Charlie found earlier,

is a pioneering species.

CRISAFULLI: It's really important

in landscapes like Mount St. Helens,

because the volcanic material that fell on the ground

tends to be really nutrient poor.

NARRATOR: The conditions are difficult,

but can the lupine pave the way for other life to follow?

NARRATOR: In the spring, geologists also return to the mountain.

During the winter months,

lava has continued to ooze out of the crater floor.

The lava dome has grown several hundred feet taller

and doubled in diameter.

It's still a hazardous place.

DZURISIN: When I stepped out of the helicopter in 1981

on the crater floor,

steam was actively rising off the growing lava dome.

There was still a tremendous amount of noise.

Rock falls were constant...

and 2,000 feet above your head

used to be where the summit of the volcano was.

You were now standing in a crater with a lava dome

that had not been there a few months previously

or a year previously.

It was actively steaming.

That was a very... very exciting thought.

NARRATOR: It's a rare opportunity to watch the process of dome building

unfold in front of their eyes.

The geologists set up instruments to monitor

what's going on,

including seismometers that can detect tremors set off by lava

as it forces its way through the rocks.

They place a series of these as close to the lava dome

as possible.

(birds chirping)

At the Cascades Volcano Observatory

in southern Washington, the seismic data pours in.

DZURISIN: The seismic record like this

records any vibration of the ground...

so we can see real rock-breaking earthquakes,

we can see rock falls.

(rumbling)

It's our job to try to understand

what all those signals mean

in terms of what the volcano might do.

NARRATOR: The first traces reflect extreme activity.

DZURISIN: Here you see the record is almost continuous--

one earthquake after the other-- bang, bang, bang.

The seismic signal is essentially continuous.

NARRATOR: The lava is breaking through rocks

and flowing across the crater floor.

Then, the seismic record reveals a cyclical pattern.

For periods of weeks to months,

earthquake activity in the crater would be pretty quiet.

NARRATOR: The lava is no longer flowing.

DZURISIN: And then a few days later, we might see a pattern like this,

more and more of these very sharp earthquakes.

NARRATOR: It's the sign of lava on the move again,

forcing its way through the round.

DZURISIN: Eventually lava would make it on to the surface,

maybe in just a couple of days,

and we're seeing a continuous record of ground shaking,

both earthquakes and rock falls.

Then, after a period of dome growth

that might last a few days or a few weeks, it goes quiet again.

That episode has ended and the pattern begins itself over.

NARRATOR: This cycle of dome building continues

for the next five years.

The pattern is so regular that when the cycle begins,

the scientists can accurately predict

what the volcano will do next.

When the first rock-breaking earthquakes occur,

they know it's only a matter of days or weeks

before the lava starts to flow again.

At one point the dome reaches nearly 1,000 feet--

almost as high as the Empire State Building.

Then, in late 1986, the seismographs go quiet.

DZURISIN: It was pretty clear

that that period of dome building had ended.

NARRATOR: But for how long?

Has the mountain gone back to sleep?

JON MAJOR: It wasn't clear whether the mountain had gone back to sleep

now for centuries

or whether it was going to just go back to sleep

for a couple of years.

NARRATOR: It seems the pattern has changed.

For six years the scientists have been able to predict

what the mountain will do next.

Now they are back to guessing if and when it will erupt again.

But even if the volcano has gone to sleep,

the wildlife continues to bounce back.

More and more gophers are spreading across

the blow-down zone.

Lupines are colonizing the pumice plain.

And what's happening at Spirit Lake is remarkable.

The May 1980 eruption obliterated all visible life

in the lake.

The surface was smothered in a blanket of debris.

In the murky water there was an explosion of bacteria.

CRISAFULLI: There were a couple of species of pneumonia

that were described,

and also the disease...

the bacteria that causes Legionnaires Disease,

legionella.

And so, many of us working in the lakes in the early days

came down with a fever.

NARRATOR: The bacteria rapidly consumed the oxygen,

making life impossible for any air-breathing organisms,

including fish, amphibians and insects.

We said it's going to be decades and decades

before this resembles anything like a typical lake

in the Cascade Mountain Range.

Well, we were surprised,

because that's not exactly what happened.

NARRATOR: Scientists begin routine water sampling.

It's a unique opportunity to see if and when life will return

from the dead.

At first there's nothing.

But as the debris settles, the water clears.

Light levels improve.

Then, three years after the eruption,

there's a crucial discovery...

microscopic plants.

They're phytoplankton-- plants that turn sunlight into oxygen.

They've been brought in by birds or blown in by the wind.

They are the basic building block of aquatic life.

Over the following months,

as light levels continue to improve,

the plankton population grows.

CRISAFULLI: In fact, between 1983 and 1986,

135 different species of these tiny plants

had colonized the lake.

They provide the oxygen and also the prey for the food web.

NARRATOR: Sunlight, oxygen and food.

Several years after its complete destruction,

Spirit Lake is coming back to life.

Four miles away, the volcano remains quiet.

The lava dome has stopped growing.

Many geologists think the show is over,

at least in their lifetime.

We had the feeling that we had probably seen our last eruption

of Mount St. Helens.

We knew there was a chance it would erupt again.

But none of us were betting on it.

NARRATOR: As the mountain sleeps, wildlife bounces back...

even in the most unexpected places.

In one of the most devastated areas of the mountain--

the pumice plain-- a gopher is seen.

It's surviving by eating lupine.

Lupines provide the food.

Gophers enrich the pumice

by burrowing their way through the ash.

They mix in fresh soil and help new plants to spread.

CRISAFULLI: When you walked around the landscape, it was those islands

created by gopher-turned soils that were very green

and full of flower and seeds.

NARRATOR: The gophers also play another role in helping wildlife spread.

Crisafulli finds a salamander in a gopher's tunnel.

CRISAFULLI: What's interesting about the gopher is they create

kilometers of underground tunnel systems.

NARRATOR: Elk are returning to the area,

helping to expand this amazing web of life.

CRISAFULLI: When elk move across the landscape,

they collapse the tunnels, creating entranceways

that salamanders and other amphibians can get access to.

And once they get beneath the ground,

these are very cool and moist sites that enable them

to survive in an otherwise inhospitable area.

And the importance of that is that it allows them to use

these underground burrows as stepping stones

during hot, dry weather and eventually to colonize

new patches of terrestrial habitat

as well as ponds and lakes.

(frog croaking)

NARRATOR: Spirit Lake now teems with amphibians.

Fish, brought to the lake by fishermen, are thriving,

a clear indication that the water quality

is returning to normal.

CRISAFULLI: What's happened with the fish was actually remarkable.

While we don't have a good handle

on the total number of fish, we know from our snorkeling

and surveys that the population is enormous.

NARRATOR: Spirit Lake is beginning to resemble

a typical mountain lake.

Just over a decade after the eruption,

life is flooding back to the slopes of Mount St. Helens.

The rate of recovery is far faster

than anybody had expected.

CRISAFULLI: Clearly our understanding of the ability of these organisms

to disperse was greatly underappreciated.

We found a lot of our conventional wisdom

was just flat wrong.

(birds chirping)

NARRATOR: Then, as life recovers, new threats emerge.

In September 2004,

the seismographs at the Cascades Volcano Observatory

pick up a new series of tremors

deep below Mount St. Helens.

The volcano has woken up.

John Pallister takes a helicopter to investigate.

PALLISTER: You could see the absolute beginning of the eruptions,

unusual-- really unusual-- to just happen to be there,

in a helicopter,

the crater rim, on the upwind side,

so the plume was going away from us.

NARRATOR: Pallister has no idea how big this eruption will be.

PALLISTER: We saw the boiling material come out of the ground.

We saw that it was blasting up.

It was dark ash coming out and light steam coming out

at the same time.

It made a plume that rose up over the rim of the caldera

and drifted downwind.

NARRATOR: The speed and suddenness of the eruption

catches everybody by surprise.

The volcano went from quiet to unrest to eruption

very, very rapidly.

(explosions)

NARRATOR: During the next two weeks, there are three more eruptions

of steam and ash.

No one knows what will happen next.

DZURISIN: It seemed possible

that we were headed toward an explosive eruption.

We didn't know.

That was a key question.

NARRATOR: Then, after 14 days, the seismographs quiet down.

Almost as quickly as it started, the eruption stops.

But then something strange happens.

Over the next few weeks, the seismographs pick up

a new pattern of tremors

the geologists have never seen before.

Could they be linked to a gigantic lump of lava

growing out of the crater floor?

PALLISTER: It was a huge kind of recumbent spine,

this big mass lying in the crater floor

some 300 meters or so high.

NARRATOR: The spine of lava is as long as the Eiffel Tower.

MAJOR: Everybody was just awestruck.

To have this large spine just shoving up out of the ground

was completely different and outside the experience

of any of us here in the staff.

NARRATOR: Despite the risk, John Pallister goes in to take samples.

PALLISTER: We landed right next to it.

And I was able to get out, helicopter helmet on,

rapidly run up to the edge of the spine.

NARRATOR: It's an unbelievable sight.

DZURISIN: Had someone suggested to me that we make a movie

of a lava dome growing that way, I think I would have said

it's a little too fantastic, let's make it more realistic.

NARRATOR: At the observatory,

where geologists have been puzzling over

the strange seismic traces, they now realize what they are.

They're the unique autograph of the giant spines

as they push their way out of the ground.

DZURISIN: This is the seismic signature of solid blocks of rock

grinding their way through the volcano,

coming out onto the surface.

As they do, they make these small seismic signals,

one just like the other, just like the other, very repetitive.

We came to call them "drumbeats."

NARRATOR: The drumbeats continue for several years.

Spine after spine of solid lava emerges from the crater floor.

It's unlike anything geologists have seen on Mount St. Helens.

PALLISTER: Now, spine doesn't do justice to these things.

These things were like skyscrapers

that were being shoved out of the ground.

They were literally that big.

NARRATOR: Sometimes the blocks grow at a rate of 16 feet a day.

Then they collapse.

Seen through a time-lapse camera,

one solid lump of lava after another

pushes up through the crater floor.

The process is mystifying.

What do the spines mean?

Why was the eruption in 2004 so different

than the style of eruption in the 1980s?

Why in the 1980s did you have this more fluid lava

that created the sort of short, stubby lava flows

that came out and built the lava dome?

Whereas in 2004, you basically had solid rock being pushed up

in the ground.

NARRATOR: There's one urgent question.

Is the volcano building up to another major eruption?

MAJOR: Trying to make sense of what was going on was a challenge.

Trying to understand how the eruption was going to progress

was a challenge.

We had lots of discussions about whether or not

it was going to be an explosive eruption,

whether it was going to be another dome building eruption.

NARRATOR: There is one way to find out.

Analyzing samples of the lava might explain

the mysterious solid blocks and what they mean for the future.

At the volcano observatory, John Pallister compares lava

from the spines with samples taken from previous eruptions.

Could there be something in their composition

that explains why the mountain sometimes pushes up spines...

sometimes oozes lava...

and sometimes explodes?

(rumbling)

Pallister begins with a sample of the lava

that erupted so explosively in May 1980.

He's immediately struck by the large areas of blue.

PALLISTER: Okay, so what's important about this 1980 rock

is the abundance of this blue area, which...

and that's basically bubbles.

Now, that's not a mineral;

that's just open space in the thin section.

That's where gas bubbles were.

This sample would float in water, it had so much gas in it.

NARRATOR: The gas comes from water, a component of the magma.

As magma rises,

changes in pressure turn the water into gas.

The gas pressurizes the magma.

It's what gives volcanoes their explosive force.

(explosions)

PALLISTER: 1980 had a lot of gas in it.

So it exploded, tore itself apart

in a tremendous explosive eruption.

NARRATOR: But when he looks at lava taken from the 1983 period

of dome building, it's different.

PALLISTER: There is much less of this open space,

of the gas filling space in the rock.

NARRATOR: The 1983 lava behaves like a bottle of soda going flat.

Finally he looks at a sample of lava

from one of the spines in 2005.

PALLISTER: I don't see any blue space,

any of that-- oh, there's just a little bit--

but dominantly it is... it is lacking in space.

It's a gas-poor magma... in fact almost none.

NARRATOR: There's just enough gas to push it to the surface.

But by the time it gets there, there's nothing left.

PALLISTER: So this one came up slow

and it made sticky, solidified spines instead of making

either lava flows or an explosive eruption.

NARRATOR: It's a crucial insight.

The amount of gas determines the nature of the lava

and the force of the eruption.

PALLISTER: It all comes down to the gas budget for the eruption.

Is it going to fizzle or is it going to explode?

NARRATOR: Suddenly the mountain's behavior makes sense.

The spines are a sign the magma under Mount St. Helens

is running low on gas.

Then, in 2007, as if to confirm this new insight,

the familiar drumbeat seismic traces...

vanish completely.

No more spines appear.

The lava below the mountain has finally run out of gas.

How long will it take the mountain to build up

enough gas pressure for another eruption?

I think that's the most important question

we have to answer.

How long does it take to build up the gas necessary

to drive an explosive eruption?

NARRATOR: That's now the question they need to answer.

Geologists go back to the mountain to look for clues.

The eruption in 1980 took the top off Mount St. Helens,

leaving its history exposed in the rock walls of the crater.

Most of the important previous eruptions are marked

by different colored bands.

The lower part of the walls where you see gray

and some yellows and some pinks

are all part of the older edifice of Mount St. Helens.

NARRATOR: These rocks, which make up the bottom half of the rock face,

are around 16,000 years old.

MAJOR: Then, if you look higher on the wall, near the top,

you see darker colors.

And those are rocks that began erupting

about 3,000 to 2,500 years ago.

So, by looking at what we call the stratigraphy

in this magnificent exposure of the rock types

in the crater walls, we can piece back the puzzle

and understand the history as far as eruptive activity

of Mount St. Helens.

NARRATOR: Do the rocks give any indication how long it takes to build up

enough gas between eruptions

for the sleepy mountain to awake again?

John Pallister sorts and categorizes rocks

from earlier eruptions.

He checks notes and photos to try and determine

how often the mountain has erupted violently.

Drawing on previous records,

he builds up a picture of Mount St. Helens' past.

For much of the last 4,000 years,

there seems to be a fairly clear pattern.

If we look at the number of big eruptions

over the length of time the volcano's been active,

you might say that there's one roughly every thousand years,

a big eruption.

So from our context here we could say

that it takes on the order of a thousand years

to build up enough gas to get a really large eruption.

NARRATOR: The record suggests some of these eruptions have been huge,

more than ten times larger than 1980,

potentially enveloping vast areas of Washington and Oregon.

But that would imply that the next really big one isn't due

for about another century.

Except for one little detail around 500 years ago.

PALLISTER: In 1479 A.D. and 1482

there were two very large eruptions.

(explosions)

So the volcano is capable of surprising us and producing

two highly explosive eruptions

in a span of less than three years.

NARRATOR: Both these eruptions were much bigger than May 1980.

There is no straightforward pattern.

Mount St. Helens can pause for a thousand years

between big explosive eruptions, or it can pause for three.

These results have left geologists with one certainty

and a number of questions.

First of all, we expect this volcano to erupt again

as repeatedly in the past;

there's no reason to think it's gone to sleep forever now.

NARRATOR: There will be another eruption,

but nobody can determine when or just how big it will be.

PALLISTER: We don't know whether it's going to erupt explosively again

n two years or in 20 years or in 200 years.

That's an area that needs a lot more work,

a lot more research to understand

and it is of fundamental importance to being able

to forecast and to save lives and to save property.

(bird screeches)

NARRATOR: For 30 years, Mount St. Helens has led scientists

on an extraordinary journey of surprise and discovery.

When they surveyed the destruction in the early 1980s,

nobody could have predicted the speed with which

life has returned.

CRISAFULLI: It was another form of an eruption,

it was an eruption of nature.

Nature marched back with a vengeance.

NARRATOR: Mount St. Helens has revealed a rich and complex web of life

that has never been documented before.

Today the slopes of the mountain are a living testimony

to the miraculous ability of nature to return from the dead.

CRISAFULLI: Each time you would come out here

and there would be a surprise, something would be unveiled,

something that you hadn't seen before.

Perhaps it would be a new species of spider

or a new species of beetle.

Nature is very resilient,

and that is the take-home message from 30 years

of ecological work on the Mount St. Helens volcano.

NARRATOR: But as nature bounces back, the mountain still broods overhead.

It's like a ticking time bomb waiting to destroy life

all over again.

MAJOR: Based on the history of this volcano,

we know it's been extremely active

and it's not a matter of whether, if it will erupt again,

it's a matter of when it will erupt again,

when will it reactivate, when will it reawaken.

NARRATOR: These are questions scientists are still wrestling with.

DZURISIN: We have yet to find a silver bullet,

a magic thing that we can measure that tells us

when the volcano is going to turn on.

NARRATOR: Mount St. Helens will erupt again.

The only questions are when and how big that eruption will be.

On NOVA's "Mount St. Helens" Web site,

go behind the scenes with director Daniel Hissen.

See stunning images

of the landscape's remarkable return to life, and more.

Find it on pbs.org.

Captioned by Media Access Group at WGBH access.wgbh.org

This NOVA program is available on DVD at shopPBS.org,

or call 1-800-play-PBS.