Avalanches 10 Things You Didn't Know About...

The great mountain ranges of the world

are some of the most breathtaking landscapes on the planet

and a holiday destination for millions,

but they are also one of the world's deadliest environments, and prime avalanche territory.

This...

This is the Alps.

Like many people, I love it here.

But it's not just the walking and the skiing that draws me here.

I've come to explore the power and the nightmare of avalanches,

and I've got some stories from around the world that might just surprise you.

So if you don't know where the world's deadliest avalanche killed 18,000 people,

which military hero lost an empire to avalanches,

or how an avalanche led to the world's greatest aviation mystery,

then stay around as I reveal 10 things you didn't know about avalanches.

It's easy to see why the awesome power of avalanches terrified our distant ancestors.

In Medieval times, it was believed that inside the Matterhorn

hung a ruined city inhabited by the souls of avalanche victims.

Avalanches were thought to be the breath of white dragons.

I'm standing in an especially beautiful corner of the Swiss Alps,

among a mountain range that's got some of the most famous peaks in Europe -

Mont Blanc, the Eiger, and the Matterhorn.

As a geologist, I spend quite a bit of time in places like this

and, inevitably, I've seen the devastation caused by avalanches.

The Alps are among the most densely populated mountain regions in the world.

That's why there are more avalanche deaths recorded here than anywhere else.

Hundreds of thousands of avalanches happen each year in the Alps alone.

I'm fascinated by the power of this force of nature.

But what few people realise is just how complex avalanches are.

These swirling masses of snow and ice are such chaotic systems

that they're incredibly difficult to comprehend.

We know more about the surface of the moon

than we do about the turbulence inside an avalanche powder cloud.

So what causes avalanches?

Skiers can certainly set them off, and it's easily done

because snow is the weakest surface material on earth.

Alpine snow is 10% ice and 90% air, making it highly unstable.

And there are many different kinds of snow.

Everyone knows that each snowflake in unique,

but it may surprise you to know that

scientists have classified these flakes into 80 different types.

Some are smooth and round, others flat and angular.

Once layers of snow fall on mountain slopes,

these physical properties determine how they bind together or fall apart

and can play a really significant part in creating avalanches.

The flat, icy flakes cause problems once they fall on mountain sides.

Without any hooks or jagged edges, they can't hold onto the layers of snow above and below,

and so they form a slippery, sliding layer in the snow pack, known as a weak layer.

Let me show you how a weak layer develops.

Imagine these two slabs of bread are two layers of snow cover.

Ordinarily, they stick together,

so if I tilt it up to quite a high angle, it's stable.

I have to tip it to...

there before it slides off.

But if a weak layer develops,

and I can represent that with these metal ball bearings, then...

The top slab slides far more easily.

And that is an avalanche.

A slab avalanche is the most common form of large avalanche caused by a weak layer collapsing.

They are deadly.

But whereas wet snow tends to form slow avalanches

that move with the consistency of wet cement,

dry snow creates an accompanying powder cloud.

These can be even more dangerous because of their power.

And that's the subject of my first story, a story about an avalanche

that happened here in the Alps and took everyone by surprise.

It's a story that shows just how unpredictable the forces of nature can be.

Mont Blanc is one of the deadliest mountains in the world.

Avalanches claim the lives of many of the climbers who come here,

but our story begins in the nearby village of Montroc,

in the shadow of this mountain of death.

In 1999, the skiing season was in full flow,

with near ideal conditions across the Alps.

But the weather changed and enough snow fell

to trigger avalanche warnings and evacuations throughout the area.

No-one from the small village of Montroc was evacuated because it was designated a safe area.

It hadn't suffered an avalanche for nearly 100 years.

But on February 9th 1999, Montroc was hit by a huge avalanche.

This avalanche was unstoppable.

It accelerated down a slope, past a plateau which would normally be enough

to stop an avalanche in its tracks, and proceeded straight towards Montroc.

It swelled to the height of a six-storey building,

and reached a speed of 60 mph,

before 40,000 tons of snow smashed through the village.

When the slide came to rest, 14 buildings had been totally destroyed

and 12 people had lost their lives.

It was as if a bomb had been dropped on the whole area.

So why did this avalanche reach the village?

The answer is a sobering reminder of just how difficult it is

for anyone to predict when an avalanche will strike and how far it will travel.

There are a number of factors that create avalanche conditions.

One is the wind.

As snow blows over ridges, it can form into dangerous cornices or overhangs.

When they fall, they can easily trigger avalanches on the slopes below.

In Montroc, the wind usually blew away from the village,

but in the days leading up to the avalanche the wind suddenly changed.

It blew in the opposite direction,

creating ledges of snow facing Montroc.

This simple switch in the direction of the wind would spell disaster.

A second factor was the snow itself.

An unusual amount of dry powder snow fell,

creating the conditions for a dry slab avalanche,

with its deadly turbo-charged powder cloud.

Dry powder avalanches behave in such an unpredictable way

that they're incredibly difficult to study,

but scientists have found a way of recreating the turbulent motion of powder clouds in miniature.

In this laboratory, tiny particles of glass

are released into tanks of water to simulate a powder avalanche.

Although they look beautiful, powder clouds can be deadly because

they can engulf their victims in a suffocating cloud of snow.

Experiments like these help to predict how far avalanches will travel.

What they reveal is that powder avalanches can build up so much momentum

that they can leap over obstacles and surge across plateaus.

They just keep going.

Surprisingly, perhaps, most people who die in avalanches don't die

from the initial impact of the snow or even from the cold.

Almost everyone who gets caught in an avalanche

can survive for up to 15 minutes completely buried in the snow.

After this, almost all victims die of asphyxiation,

breathing in carbon dioxide trapped in the air pocket around them.

Very few last long enough to succumb to hypothermia.

Death by avalanche isn't called "white death" for nothing.

This extraordinary footage captured the moment

three skiers were caught in an avalanche of their own making.

In fact, 90% of people caught in avalanches

actually start them themselves.

In this situation, creating a breathing space

could be the difference between life and death.

When the avalanche stops, the snow sets like concrete in seconds.

The skiers filmed in this avalanche survived, but others are not so lucky.

The villagers of Montroc had no chance.

They were helpless in the face of a combination

of these unpredictable forces of nature; the wind, weather and snow.

I first learned of my next story when I was a kid.

For some, it's a story of epic folly but for me, it's just plain epic.

It's a tale of hardship and heroism,

of remarkable risk-taking that took place in mountains 2,000 years ago.

And it's the only story I know in which avalanches may have changed the course of human history.

Long ago, when the world was in the grip of Rome,

one legendary general decided to strike at the very heart of the Roman Empire.

That man was Hannibal.

Rome and Hannibal's home of Carthage were at war.

Hannibal knew his only chance of success against his enemy's mighty armies

was to take them by surprise, so he devised a cunning plan.

He decided to march his army over the Alps in the dead of winter,

when no-one would be expecting them.

His army of 38,000 soldiers, 8,000 horsemen and 37 elephants

would suffer brutal weather when they crossed first the Pyrenees

and then the Alps on their journey to Rome.

Freezing weather, inadequate clothing and perilous climbs

all took their toll on his troops and elephants.

But what actually devastated Hannibal and his army

was something they hadn't expected at all - avalanches.

During the three months that it took them to march into Italy,

18,000 soldiers and 2,000 cavalry men died -

many were swept away by avalanches.

The Roman poet Silius Italicus later described what happened

in one of the first documented avalanches in history...

By the time Hannibal and his army descended into the foothills of northern Italy,

40% of his soldiers were dead and those who had survived were ravaged and exhausted.

With his depleted troops, it's hardly surprising that

now Hannibal was unable to defeat the Roman Empire.

Avalanches had seriously damaged his chances of changing history.

A sad end to his epic journey.

I guess everyone appreciates that avalanches are incredibly destructive.

But perhaps not many realise that, at times, they were used to kill deliberately.

And it was in the First World War that avalanches were first used as weapons of mass destruction.

During the Great War, the Alps became a fierce battleground.

Thousands of soldiers were lost as they fought in high mountain passes.

But what's really surprising is that over 60,000 men were killed by avalanches.

The mountains themselves were a deadly enemy,

and that's the subject of my next story.

It was mostly Italian and Austro-Hungarian soldiers that fought here.

Many of the troops were specialist mountain units

trained to ski, climb and fight in this dangerous terrain.

But others were drafted in from the lowlands

and were totally unprepared for the extreme conditions.

A visiting British soldier described the Italian camp:

"Most of the men in the camp were very young, thinly clad and feeling the cold intensely,

"and they had been left in their line for a long period without relief.

"Many of them were weeping and some had ice on their faces.

"The conducting officer said that three or four of them were frozen to death nightly."

Both forces dug tunnels into the mountain to avoid enemy shells.

They dragged huge guns into fortified positions at heights of up to 3,500 metres.

These guns dominated entire valleys, making an attack

up the steep, rugged slopes dangerous and frightening.

But the most terrifying enemy was nature itself.

In just one night in December 1916, after days of heavy snowfall,

105 avalanches thundered across the battle lines.

10,000 men died in this night of horror.

But these colossal casualties actually gave the generals a new and deadly tactic.

Both sides realised that they could turn avalanches into weapons

by deliberately triggering them to strike enemy lines and bury the soldiers alive.

So the guns were moved and pointed in a new direction.

They were no longer aimed directly at the enemy, but at the ridges of snow above them.

A single round could dislodge an entire Alpine snowfield,

sending an avalanche hurtling down the mountainside, onto the enemy below.

Entire platoons were killed, without a single shot being fired in return.

In the battle for the Alpine front, the power of nature proved to be

a far more potent weapon than anything man could invent.

Even today, the remains of soldiers who fought in the Alps during World War I

are still being discovered as the glaciers on the mountain retreat.

These two Austrian soldiers were dug from the ice in 2004, still in their uniform.

In the end, whether natural or man-made,

avalanches claimed the lives of thousands of soldiers.

To use avalanches as a weapon of war was a brutal tactic,

but then again, it was a brutal war.

It's not just harsh winters that cause avalanches -

it looks like global warming could be triggering them as well.

As the climate changes, snow and ice on the mountains,

and glaciers like that over there, become increasingly unpredictable.

Some believe that global warming contributed to the collapse of the Kolka glacier in Russia in 2002.

The collapse produced a huge avalanche that swept away an entire village.

It was the worst avalanche disaster in Russian history.

A glacier is a slow moving river of ice,

formed by compacted snow laid down over thousands of years.

Once it reaches a critical thickness, around 10 metres,

the ice becomes so heavy that it begins to move,

pulled down by the force of gravity.

Glaciers are known to advance and retreat, but usually only a matter of metres a year.

But in the Caucasus Mountains in Russia,

an avalanche triggered the Kolka glacier to move on a different scale altogether.

On the night of September 20th 2002, a wall of ice collapsed from the steep mountain above.

It created a small, but powerful avalanche.

This avalanche smashed into the Kolka glacier.

Seismometers, usually used to record the size of earthquakes,

registered the impact.

The impact of the avalanche hitting the glacier was so great the unimaginable happened.

Incredibly, the entire Kolka glacier broke free.

While it normally moved at a snail's pace,

now the entire glacier raced down the mountain at over 100 miles an hour.

Millions of tons of ice and rock surged towards the settlements below.

Just five minutes and 15 miles later, it reached the village of Karmadon.

Within seconds, the village was engulfed in ice, rock and mud.

No-one in its path stood a chance.

Over 120 people died.

This was a unique geological event and the largest ice avalanche ever documented.

Scientists now believe that the first small avalanche,

which happened high above the Kolka glacier

may have melted and weakened due to global warming.

And this kind of avalanche may happen more frequently

as the earth's atmospheric temperature increases.

But that doesn't explain the extraordinary collapse of the entire Kolka glacier.

Some scientists now think that volcanic activity

may have warmed the ground beneath the glacier,

softening the bottom layer of ice;

so when an avalanche smashed into it,

the glacier was already primed for destruction.

What we do know is that global warming makes snow and ice in mountain regions

far more unstable and less predictable,

and this might well create more avalanches in the future than we ever would have expected.

My next avalanche is a story seldom told,

but it's the worst avalanche that ever hit America.

The town that it struck no longer exists because it never recovered from the calamity that befell it.

What occurred in the Cascade Mountains in 1910

was a far greater tragedy than it should ever have been,

because of one terrible coincidence.

The avalanche happened to strike the exact spot

where two trains were trapped in the snow.

The Great Northern Railway in the United States

passes through the Cascade Mountains in Washington State.

There was once a small town here called Wellington.

They were used to freezing weather here

and were well-equipped to deal with it.

Tunnels kept the worst of the snow off the tracks,

and they had ploughs to clear the snow that remained.

But these defences were powerless in the face of a terrible blizzard

that struck the area in February 1910.

For nine whole days it raged, with a foot of snow falling each hour.

The snowfall was so heavy that the track was closed,

leaving two trains bound for Seattle stuck fast outside Wellington;

a passenger train and a mail train.

For five days, the trains were trapped by 25ft drifts.

High above on Windy Mountain, the snow piled up.

The only relief for the passengers and crew

was to trek into Wellington for something to eat and to buy tobacco.

But then they returned to the carriages to sleep every night.

What bad luck, then, that the avalanche struck at night.

Shortly after midnight on March 1st 1910 came a violent thunderstorm.

Dislodged by the storm, or by its own mass,

a huge swathe of cement-like snow

now poured down the mountain towards the carriages below.

A 10 foot wall of snow, half a mile wide, raced towards the trains.

The passengers fast asleep in the carriages had no idea what was coming.

They were swept violently into the air and thrown 150 feet down into the valley below.

Steam hissed from the wreckage of the train.

The snow was red with blood.

96 men, women and children died, in America's worst avalanche disaster.

Astonishingly, 23 passengers were rescued alive.

But when every survivor had been pulled from the wreckage,

the grim task of collecting the bodies was abandoned because the conditions were so bad.

It wasn't until the following summer that the last of the dead were finally retrieved.

Surprisingly, it may have been the rain that caused this catastrophic avalanche.

Wet snow has less strength than dry snow.

Water weakens the bonds between the ice crystals

and the added weight reduces the stability of the snow pack.

Warm air from the Pacific had travelled to the Cascade Mountains,

creating conditions for a thunder and lightning storm.

And the warmer temperature, together with the rain that followed,

would have loosened the snow and made it more susceptible to collapse.

All these factors almost certainly combined

to dislodge a vast slab of snow on that fateful night in Wellington -

to this day, America's worst avalanche disaster.

My next story takes us back to the winter of 1951,

when Europe was experiencing some of its harshest weather.

The Alps were struck by thousands of avalanches.

It was the worst winter in recorded history and dubbed the Winter of Terror.

But, in a way, out of that terror came hope because things got so bad in 1951 that all the Alpine nations

came together to do something about it, and we are still reaping the safety benefits from that today.

No-one was prepared for the scale of misery that descended on the Alps during the Winter of Terror in 1951.

Over the course of just a few months, thousands of avalanches struck, killing over 200 people.

Roads were blocked, leaving communities stranded without supplies

and unable to escape the danger zones.

The relentless tragedies meant that when an avalanche struck, everyone would help with the rescue efforts.

It soon became clear that it was impossible to get to everyone in time.

But this was also a turning point.

Things got so bad that it prompted all the nations of the Alps

to dramatically increase the measures taken to protect their citizens.

They turned to the first dedicated Avalanche Research Institute at Davos, Switzerland,

who began to focus their attention on devising new ways of protecting people from avalanches.

Having studied the physics and mechanics of snow,

their knowledge was now applied to the causes of avalanches - how to predict them and prevent them.

They examined snow crystals, working out how they affect the stability of the snow pack,

and they studied weak layers under the microscope

to see how they are influenced by different weather conditions.

We're still reaping the benefits today from what they learned.

The Winter of Terror led to the creation of new kinds of avalanche defences -

rows of fences to hold back the snow,

as well as rescue services and avalanche zoning to show safe and dangerous areas.

But there was another crucial thing that came out of the Winter of Terror -

it was now that people took up the practice of starting avalanches deliberately.

Not to kill, but to save lives.

And this has become the single most important part of avalanche safety we have.

There is now an army of about 2,000 men and women

who deliberately start avalanches in the most dangerous places in the Alps.

They're adrenaline junkies. They like to blow things up, and their fix is blasting mountain slopes.

And the government pays them sometimes as little as just £15 a day danger money to do it.

For that pay, I guess you just have to love the job!

Any slope that looks like it might be about to cause an avalanche is cleared of skiers,

then these intrepid snow blasters turn up - often by helicopter - to cause a controlled avalanche

that clears the snow safely so that large slab avalanches don't develop and threaten the slopes below.

Of course, it's not safe for them.

This highly skilled job is extremely dangerous.

Their tool kit consists of rope, fuse and dynamite.

Dynamite is attached to a cord to prevent it sliding too far down the slope and causing danger below.

If the dynamite doesn't detonate the first time, they've no option but to reel it in and try again.

But their greatest challenge lies in knowing how to get away from the avalanche they've created.

That's quite a skill.

Thanks to people like this who risk their lives for us, people can ski here more safely,

and all because some human snow-blasters are prepared to clear the slopes with dynamite.

My next avalanche happened in the Andes of South America,

and it led to one of the most bizarre aviation mysteries of all time.

The avalanche caused an entire aeroplane to vanish.

It mysteriously reappeared 50 years later, complete with its passengers and crew.

This story starts just after the Second World War, before the age of commercial jet travel,

at a time when the planes that passengers travelled in remind me of scenes from the movie Casablanca.

On August 2nd 1947, a converted Lancaster bomber, the Stardust,

took off on a regular passenger flight across South America.

But the flight itself proved to be anything but routine.

It should have taken four hours to travel from Buenos Aires in Argentina

to the Chilean capital Santiago across the Andes Mountains.

At the controls was a highly experienced pilot.

Some of the six passengers on board seemed to have stepped straight out

of an Agatha Christie novel - a Palestinian businessman with a large diamond sewn into his jacket,

a German emigre returning to Chile with the ashes of her dead husband,

and a British King's messenger, apparently carrying vital diplomatic correspondence.

But no-one on board was ever to reach their destination.

Just before Stardust was due to land, the plane sent a mysterious Morse Code message -

S-T-E-N-D-E-C.

Baffled by this unintelligible word, the radio operator in Santiago asked for clarification.

The same word - S-T-E-N-D-E-C - was repeated twice more and, after that, nothing more was heard.

The plane simply vanished.

The search began, but there was no sign of Stardust around Santiago,

so the search spread out to cover the Andes mountains.

Aeroplanes criss-crossed a vast area, but the searchers found nothing.

The plane had completely disappeared.

And so began one of aviation's most enduring mysteries.

Rumours were rife -

the plane had been blown up because of the documents carried by the King's messenger,

alien abduction was suggested, and just what did S-T-E-N-D-E-C mean?

What we would eventually find out was even more surprising.

In the year 2000, a mysterious discovery reopened the case of the vanishing plane.

An old Rolls Royce engine was found lying on a glacier high in the Andes mountains.

It had appeared out of nowhere, and it belonged to Stardust.

Over the last 50 years,

the area had been frequently visited by mountaineers and they'd found nothing.

So why had the wreckage, together with human remains, suddenly appeared out of nowhere?

It took the combined efforts of an air crash investigator and a glaciologist to solve the mystery

and, believe me, it's better than any Agatha Christie story.

All those years ago, in 1947,

Stardust had been flying over the mountains in bad weather and strayed hopelessly off course.

Miscalculating their position, the pilot thought they were near Santiago

and began their descent to land.

But it wasn't the runway ahead - it was Mount Tupungato, one of the highest mountains in the Andes,

and they crashed straight into it.

The impact of the crash set off a massive avalanche that completely covered the plane.

The wreckage was now totally hidden from the view of aircraft looking for the crash site.

Stardust disappeared under the avalanche,

but what made the plane vanish for 50 years was that it was swallowed up by the glacier.

So how exactly can a glacier swallow a plane?

Year by year, layers of snow would have buried the wreckage deeper and deeper inside the glacier.

Gradually, Stardust would become part of the glacier itself,

travelling slowly downhill - not on the surface of the ice, but deep inside.

It would move no more than a few metres a year

until, 53 years later in 2000, the plane and its passengers emerged at the bottom of the glacier.

The avalanche that covered the plane had merely started this extraordinary vanishing act.

So the mystery was solved. Well, kind of.

To this day, no-one understands what was meant by the final Morse Code message - S-T-E-N-D-E-C.

So a piece of the puzzle remains missing about the story of Stardust.

My next story is about an avalanche that happened in Austria.

This wasn't only a human tragedy, it was also a scientific mystery -

a mystery that was only finally solved in one of the most unusual avalanche experiments ever.

Somehow scientists managed to put themselves inside an avalanche whilst it was happening.

They all lived to tell the tale, and the evidence that they gathered

revealed something completely new about avalanches.

Galtur is a small village in Austria, close to the Swiss border.

Like many other towns in the Alps, this was designated a safe area,

built far enough away from the base of the mountain that no avalanche should ever reach it.

In the past, a small avalanche followed the same route each year,

but trickled out safely at the bottom of the slope.

Computer models had indicated that no normal avalanche would have the power to reach the village,

over 200 metres from the foot of the mountain.

And yet, like too many apparently safe towns, the unexpected happened here in 1999

when it was hit by a massive avalanche.

In only 50 seconds, the avalanche raced down the mountain

and crossed a flat valley that should have stopped it dead.

But this avalanche didn't stop.

It kept on going across the valley and hit the village at a speed of almost 200 mph.

31 people died.

It was described by survivors as being like a huge Tsunami.

This is not the first time an avalanche has defied expectations

and struck in a place no avalanche should reach.

Scientists had been determined to understand why this kept happening.

What was missing from their computer models, from their understanding of avalanches,

that meant they were underestimating the potential power of certain avalanches?

The scientists at Davos had studied the physics of snow for decades under laboratory conditions,

but they realised that there must be something more going on inside an avalanche that they had never seen.

They were determined to work out what it was.

But instead of bringing more snow to the lab, they took the lab to the snow,

placing their instruments and themselves inside an avalanche.

But how could they get right inside the lethal onslaught of an avalanche while it was in full flow?

They came up with an ingenious solution.

They built a reinforced concrete bunker smack in the middle of avalanche territory.

Then, in February 1999, the scientists put themselves

and a mass of scientific equipment including radar, pressure monitors and flow measures into the bunker,

locked the heavy iron doors behind them

and waited.

Meanwhile, far above them, a massive avalanche was deliberately set off with explosives.

It was directed straight at them.

As snow thundered down the mountain, the bunker was completely engulfed.

They hadn't anticipated quite how big the avalanche would be.

We were surprised by the force by which the avalanche hit the shelter.

We did not expect such a big thing to come down.

This is a very massive and solid bunker - I think the walls are about 40 centimetres thick -

and you could feel the vibrations of the whole building.

We heard a strong noise.

That was because the door broke open and the snow came in.

Then the pressure in the shelter rose enormously.

It was like diving into two metres of water.

After the avalanche had hit us, we had first to try to get out,

because the shelter was completely covered by snow.

And so we started digging a tunnel out, and then we had to work hard.

It was, really, very compact snow.

So, after all this, what had they discovered?

Radar allowed the scientists to peer inside each layer of snow as the avalanche was happening.

This revealed a critical and deadly layer of snow that no-one had understood before.

It's called the saltation layer - a band of dense, heavy snow in the middle of the avalanche flow.

Scientists discovered that this saltation layer is what can turbo-charge an avalanche,

boosting its power and speed.

As the heavy snow particles in the saltation layer bounce around within the avalanche,

they gather up new snow, dramatically increasingly its volume and power

so that the whole mass plummets down a slope with unstoppable force.

Based on this research, these scientists were now able

to explain how the Galtur avalanche reached the town.

It was the saltation layer that allowed the avalanche

to build up energy and momentum and so travel across the flat valley floor and destroy the village.

Thanks to the discovery in the bunker, today the equations for the saltation layer

are routinely added to computer models to assess avalanche risks throughout the world.

Since Galtur, the bunker experiment has been performed many times,

and each time it reveals something new about avalanches.

Scientists have come to realise that in order to really understand how nature works,

they've got to be prepared to put themselves in the line of fire.

Our next avalanche is the worst in history.

It happened in Peru and it killed more people than any other avalanche we know of.

It was enormous, and that's because it was triggered by a massive earthquake.

South America boasts some of the world's most stunning landscapes,

but its unstable geology also makes it an unpredictable and deadly area.

Earthquakes and volcanoes strike without warning.

May 31st 1970, the opening day of the football World Cup.

Soccer is Peru's national sport and because the country had qualified,

people were out in the streets of virtually every town and village, having a fiesta.

But while the celebrations were in full swing, disaster was waiting in the wings.

Deep in the Pacific ocean, 20 miles off Peru's coast, the seafloor ruptured.

A massive earthquake sent shock waves racing towards Peru's mainland.

Within seconds, the first shock waves hit the coast,

and soon reached the snow-capped peaks of the Peruvian Andes.

As the shock waves hit Mount Huascaran, the highest mountain in Peru,

it triggered an enormous glacial avalanche.

What is so significant about this avalanche is its sheer size.

This one was a mile long and half a mile wide.

Travelling at over 100 miles per hour,

it launched the most lethal and destructive avalanche the world has ever seen.

It raced down the 20,000 foot mountain, picking up rocks and debris as it went.

It took just a few minutes for the deadly mass of snow, water,

mud and rock to travel ten miles, all the way to the town of Yungay.

It engulfed the entire settlement, covering it in mud many metres deep.

It's believed that 18,000 people perished in the town that day.

Yungay became a ghostly place.

The tops of four palm trees were all that remained of its central square.

No-one knows exactly how many bodies are buried here.

The Peruvian government has forbidden the excavation of Yungay, declaring it a national cemetery,

and so much of the area remains eerily as it was

when the most deadly avalanche in history wiped out this town.

My final avalanche is the most terrifying monster of all.

This particular kind of avalanche has rarely been filmed or photographed,

because anyone who sees them is usually killed by them.

They're are the most deadly avalanche of all -

rock avalanches.

We usually think of avalanches as tumbling snow,

but actually anything that flows at high speed is called an avalanche.

And what's amazing is that under certain rare conditions,

even rock can behave like snow, cascading down a mountainside just like a snow avalanche.

I'm not talking about a landslide.

This is a landslide - a slow-moving mass of rock and earth pulled down by gravity.

But just take a look at this.

Although it looks like a river of water, it's just rock and dust.

As huge pieces of rock break free from the mountain and cascade down a slope,

the rocks break up into smaller and smaller pieces.

Fine dust particles change the dynamics of the flow,

the physics of which are extremely complex, just like snow avalanches.

This is a controlled experiment by scientists - trying to film the real thing happening is too dangerous.

In these experiments, scientists use rock debris to demonstrate how rock behaves in an avalanche.

Exactly how rocks able to flow and move so far is still not clear.

Some scientists suggest that the avalanche flows on a bed of air,

others say that when the rocks hit the valley bottom they explode, releasing stored energy.

The simple answer is we still don't know.

Rock avalanches are very rare, and it's even rarer to actually witness one.

That's why scientists still know so little about them.

But we do know that it is this flowing property

that means the rocks don't stop when they reach the valley floor.

They flow out over the surrounding area, just like a snow avalanche,

threatening towns and villages far away from the slope.

That's what happened to the town of Frank in Canada.

In 1903, it was hit by a rock avalanche which struck without warning.

90 million tonnes of rock fell from the summit of Turtle Mountain,

and when it hit the ground, it spread over a two mile area of the valley in less than two minutes.

Survivors in the town of Frank recalled hearing a terrible sound

like cannon fire echoing around the valley as the mountain collapsed.

70 of the 600 inhabitants were killed.

Most of the bodies were never recovered -

they were buried beneath the rubble.

Despite their unpredictability, we can expect more of these terrifying rock avalanches in the future.

Scientists can now identify the danger zones because they've studied unstable slopes around the world.

What we know is that, although they'll always be rare,

these extraordinary rock avalanches will happen again - with lethal consequences.

So we come to the end of our ten remarkable stories about avalanches.

If we choose to live and work in the shadow of mountains like these,

we're always gonna be at risk from avalanches.

No matter how safe things might appear and despite all the advances of modern science,

nature will always have the last word.