0:00:07 > 0:00:09The great mountain ranges of the world
0:00:09 > 0:00:13are some of the most breathtaking landscapes on the planet
0:00:13 > 0:00:16and a holiday destination for millions,
0:00:16 > 0:00:21but they are also one of the world's deadliest environments, and prime avalanche territory.
0:00:39 > 0:00:41This...
0:00:41 > 0:00:43This is the Alps.
0:00:45 > 0:00:48Like many people, I love it here.
0:00:48 > 0:00:52But it's not just the walking and the skiing that draws me here.
0:00:52 > 0:00:57I've come to explore the power and the nightmare of avalanches,
0:00:57 > 0:01:01and I've got some stories from around the world that might just surprise you.
0:01:10 > 0:01:16So if you don't know where the world's deadliest avalanche killed 18,000 people,
0:01:16 > 0:01:20which military hero lost an empire to avalanches,
0:01:20 > 0:01:24or how an avalanche led to the world's greatest aviation mystery,
0:01:24 > 0:01:31then stay around as I reveal 10 things you didn't know about avalanches.
0:01:48 > 0:01:55It's easy to see why the awesome power of avalanches terrified our distant ancestors.
0:01:55 > 0:02:00In Medieval times, it was believed that inside the Matterhorn
0:02:00 > 0:02:05hung a ruined city inhabited by the souls of avalanche victims.
0:02:05 > 0:02:10Avalanches were thought to be the breath of white dragons.
0:02:10 > 0:02:16I'm standing in an especially beautiful corner of the Swiss Alps,
0:02:16 > 0:02:21among a mountain range that's got some of the most famous peaks in Europe -
0:02:21 > 0:02:24Mont Blanc, the Eiger, and the Matterhorn.
0:02:24 > 0:02:28As a geologist, I spend quite a bit of time in places like this
0:02:28 > 0:02:33and, inevitably, I've seen the devastation caused by avalanches.
0:02:38 > 0:02:42The Alps are among the most densely populated mountain regions in the world.
0:02:42 > 0:02:47That's why there are more avalanche deaths recorded here than anywhere else.
0:02:47 > 0:02:52Hundreds of thousands of avalanches happen each year in the Alps alone.
0:02:55 > 0:02:58I'm fascinated by the power of this force of nature.
0:02:58 > 0:03:03But what few people realise is just how complex avalanches are.
0:03:16 > 0:03:21These swirling masses of snow and ice are such chaotic systems
0:03:21 > 0:03:23that they're incredibly difficult to comprehend.
0:03:25 > 0:03:28We know more about the surface of the moon
0:03:28 > 0:03:32than we do about the turbulence inside an avalanche powder cloud.
0:03:37 > 0:03:39So what causes avalanches?
0:03:39 > 0:03:44Skiers can certainly set them off, and it's easily done
0:03:44 > 0:03:48because snow is the weakest surface material on earth.
0:03:48 > 0:03:55Alpine snow is 10% ice and 90% air, making it highly unstable.
0:03:58 > 0:04:01And there are many different kinds of snow.
0:04:01 > 0:04:03Everyone knows that each snowflake in unique,
0:04:03 > 0:04:05but it may surprise you to know that
0:04:05 > 0:04:10scientists have classified these flakes into 80 different types.
0:04:12 > 0:04:16Some are smooth and round, others flat and angular.
0:04:16 > 0:04:20Once layers of snow fall on mountain slopes,
0:04:20 > 0:04:25these physical properties determine how they bind together or fall apart
0:04:25 > 0:04:30and can play a really significant part in creating avalanches.
0:04:30 > 0:04:36The flat, icy flakes cause problems once they fall on mountain sides.
0:04:36 > 0:04:42Without any hooks or jagged edges, they can't hold onto the layers of snow above and below,
0:04:42 > 0:04:48and so they form a slippery, sliding layer in the snow pack, known as a weak layer.
0:04:49 > 0:04:52Let me show you how a weak layer develops.
0:04:52 > 0:04:56Imagine these two slabs of bread are two layers of snow cover.
0:04:56 > 0:04:58Ordinarily, they stick together,
0:04:58 > 0:05:01so if I tilt it up to quite a high angle, it's stable.
0:05:01 > 0:05:03I have to tip it to...
0:05:03 > 0:05:05there before it slides off.
0:05:05 > 0:05:06But if a weak layer develops,
0:05:06 > 0:05:12and I can represent that with these metal ball bearings, then...
0:05:15 > 0:05:18The top slab slides far more easily.
0:05:18 > 0:05:20And that is an avalanche.
0:05:25 > 0:05:31A slab avalanche is the most common form of large avalanche caused by a weak layer collapsing.
0:05:31 > 0:05:34They are deadly.
0:05:34 > 0:05:37But whereas wet snow tends to form slow avalanches
0:05:37 > 0:05:41that move with the consistency of wet cement,
0:05:41 > 0:05:45dry snow creates an accompanying powder cloud.
0:05:45 > 0:05:49These can be even more dangerous because of their power.
0:05:53 > 0:05:59And that's the subject of my first story, a story about an avalanche
0:05:59 > 0:06:02that happened here in the Alps and took everyone by surprise.
0:06:02 > 0:06:08It's a story that shows just how unpredictable the forces of nature can be.
0:06:14 > 0:06:19Mont Blanc is one of the deadliest mountains in the world.
0:06:19 > 0:06:24Avalanches claim the lives of many of the climbers who come here,
0:06:24 > 0:06:27but our story begins in the nearby village of Montroc,
0:06:27 > 0:06:30in the shadow of this mountain of death.
0:06:40 > 0:06:44In 1999, the skiing season was in full flow,
0:06:44 > 0:06:48with near ideal conditions across the Alps.
0:06:48 > 0:06:51But the weather changed and enough snow fell
0:06:51 > 0:06:56to trigger avalanche warnings and evacuations throughout the area.
0:06:59 > 0:07:07No-one from the small village of Montroc was evacuated because it was designated a safe area.
0:07:07 > 0:07:11It hadn't suffered an avalanche for nearly 100 years.
0:07:13 > 0:07:20But on February 9th 1999, Montroc was hit by a huge avalanche.
0:07:32 > 0:07:34This avalanche was unstoppable.
0:07:34 > 0:07:40It accelerated down a slope, past a plateau which would normally be enough
0:07:40 > 0:07:45to stop an avalanche in its tracks, and proceeded straight towards Montroc.
0:07:45 > 0:07:48It swelled to the height of a six-storey building,
0:07:48 > 0:07:51and reached a speed of 60 mph,
0:07:51 > 0:07:56before 40,000 tons of snow smashed through the village.
0:08:11 > 0:08:16When the slide came to rest, 14 buildings had been totally destroyed
0:08:16 > 0:08:20and 12 people had lost their lives.
0:08:20 > 0:08:24It was as if a bomb had been dropped on the whole area.
0:08:25 > 0:08:29So why did this avalanche reach the village?
0:08:29 > 0:08:33The answer is a sobering reminder of just how difficult it is
0:08:33 > 0:08:39for anyone to predict when an avalanche will strike and how far it will travel.
0:08:39 > 0:08:43There are a number of factors that create avalanche conditions.
0:08:43 > 0:08:44One is the wind.
0:08:44 > 0:08:50As snow blows over ridges, it can form into dangerous cornices or overhangs.
0:08:50 > 0:08:56When they fall, they can easily trigger avalanches on the slopes below.
0:08:56 > 0:09:00In Montroc, the wind usually blew away from the village,
0:09:00 > 0:09:05but in the days leading up to the avalanche the wind suddenly changed.
0:09:05 > 0:09:07It blew in the opposite direction,
0:09:07 > 0:09:11creating ledges of snow facing Montroc.
0:09:11 > 0:09:17This simple switch in the direction of the wind would spell disaster.
0:09:17 > 0:09:20A second factor was the snow itself.
0:09:20 > 0:09:23An unusual amount of dry powder snow fell,
0:09:23 > 0:09:26creating the conditions for a dry slab avalanche,
0:09:26 > 0:09:30with its deadly turbo-charged powder cloud.
0:09:38 > 0:09:42Dry powder avalanches behave in such an unpredictable way
0:09:42 > 0:09:46that they're incredibly difficult to study,
0:09:46 > 0:09:52but scientists have found a way of recreating the turbulent motion of powder clouds in miniature.
0:09:56 > 0:09:59In this laboratory, tiny particles of glass
0:09:59 > 0:10:04are released into tanks of water to simulate a powder avalanche.
0:10:04 > 0:10:08Although they look beautiful, powder clouds can be deadly because
0:10:08 > 0:10:13they can engulf their victims in a suffocating cloud of snow.
0:10:13 > 0:10:19Experiments like these help to predict how far avalanches will travel.
0:10:19 > 0:10:23What they reveal is that powder avalanches can build up so much momentum
0:10:23 > 0:10:27that they can leap over obstacles and surge across plateaus.
0:10:27 > 0:10:30They just keep going.
0:10:38 > 0:10:42Surprisingly, perhaps, most people who die in avalanches don't die
0:10:42 > 0:10:47from the initial impact of the snow or even from the cold.
0:10:47 > 0:10:49Almost everyone who gets caught in an avalanche
0:10:49 > 0:10:54can survive for up to 15 minutes completely buried in the snow.
0:10:54 > 0:10:58After this, almost all victims die of asphyxiation,
0:10:58 > 0:11:02breathing in carbon dioxide trapped in the air pocket around them.
0:11:02 > 0:11:07Very few last long enough to succumb to hypothermia.
0:11:07 > 0:11:11Death by avalanche isn't called "white death" for nothing.
0:11:19 > 0:11:22This extraordinary footage captured the moment
0:11:22 > 0:11:25three skiers were caught in an avalanche of their own making.
0:11:25 > 0:11:29In fact, 90% of people caught in avalanches
0:11:29 > 0:11:31actually start them themselves.
0:11:42 > 0:11:46In this situation, creating a breathing space
0:11:46 > 0:11:48could be the difference between life and death.
0:11:51 > 0:11:56When the avalanche stops, the snow sets like concrete in seconds.
0:11:56 > 0:12:01The skiers filmed in this avalanche survived, but others are not so lucky.
0:12:01 > 0:12:04The villagers of Montroc had no chance.
0:12:04 > 0:12:06They were helpless in the face of a combination
0:12:06 > 0:12:12of these unpredictable forces of nature; the wind, weather and snow.
0:12:21 > 0:12:24I first learned of my next story when I was a kid.
0:12:24 > 0:12:30For some, it's a story of epic folly but for me, it's just plain epic.
0:12:30 > 0:12:32It's a tale of hardship and heroism,
0:12:32 > 0:12:38of remarkable risk-taking that took place in mountains 2,000 years ago.
0:12:38 > 0:12:44And it's the only story I know in which avalanches may have changed the course of human history.
0:12:53 > 0:12:57Long ago, when the world was in the grip of Rome,
0:12:57 > 0:13:02one legendary general decided to strike at the very heart of the Roman Empire.
0:13:02 > 0:13:05That man was Hannibal.
0:13:07 > 0:13:10Rome and Hannibal's home of Carthage were at war.
0:13:12 > 0:13:18Hannibal knew his only chance of success against his enemy's mighty armies
0:13:18 > 0:13:22was to take them by surprise, so he devised a cunning plan.
0:13:22 > 0:13:27He decided to march his army over the Alps in the dead of winter,
0:13:27 > 0:13:29when no-one would be expecting them.
0:13:36 > 0:13:43His army of 38,000 soldiers, 8,000 horsemen and 37 elephants
0:13:43 > 0:13:47would suffer brutal weather when they crossed first the Pyrenees
0:13:47 > 0:13:51and then the Alps on their journey to Rome.
0:13:51 > 0:13:55Freezing weather, inadequate clothing and perilous climbs
0:13:55 > 0:13:59all took their toll on his troops and elephants.
0:14:03 > 0:14:07But what actually devastated Hannibal and his army
0:14:07 > 0:14:12was something they hadn't expected at all - avalanches.
0:14:22 > 0:14:25During the three months that it took them to march into Italy,
0:14:25 > 0:14:3018,000 soldiers and 2,000 cavalry men died -
0:14:30 > 0:14:34many were swept away by avalanches.
0:14:34 > 0:14:39The Roman poet Silius Italicus later described what happened
0:14:39 > 0:14:42in one of the first documented avalanches in history...
0:15:05 > 0:15:11By the time Hannibal and his army descended into the foothills of northern Italy,
0:15:11 > 0:15:1840% of his soldiers were dead and those who had survived were ravaged and exhausted.
0:15:18 > 0:15:21With his depleted troops, it's hardly surprising that
0:15:21 > 0:15:26now Hannibal was unable to defeat the Roman Empire.
0:15:31 > 0:15:36Avalanches had seriously damaged his chances of changing history.
0:15:36 > 0:15:39A sad end to his epic journey.
0:15:41 > 0:15:46I guess everyone appreciates that avalanches are incredibly destructive.
0:15:46 > 0:15:52But perhaps not many realise that, at times, they were used to kill deliberately.
0:15:52 > 0:15:59And it was in the First World War that avalanches were first used as weapons of mass destruction.
0:16:05 > 0:16:09During the Great War, the Alps became a fierce battleground.
0:16:09 > 0:16:15Thousands of soldiers were lost as they fought in high mountain passes.
0:16:15 > 0:16:21But what's really surprising is that over 60,000 men were killed by avalanches.
0:16:21 > 0:16:24The mountains themselves were a deadly enemy,
0:16:24 > 0:16:27and that's the subject of my next story.
0:16:39 > 0:16:44It was mostly Italian and Austro-Hungarian soldiers that fought here.
0:16:45 > 0:16:49Many of the troops were specialist mountain units
0:16:49 > 0:16:53trained to ski, climb and fight in this dangerous terrain.
0:16:55 > 0:16:58But others were drafted in from the lowlands
0:16:58 > 0:17:02and were totally unprepared for the extreme conditions.
0:17:04 > 0:17:08A visiting British soldier described the Italian camp:
0:17:10 > 0:17:16"Most of the men in the camp were very young, thinly clad and feeling the cold intensely,
0:17:16 > 0:17:20"and they had been left in their line for a long period without relief.
0:17:20 > 0:17:24"Many of them were weeping and some had ice on their faces.
0:17:24 > 0:17:29"The conducting officer said that three or four of them were frozen to death nightly."
0:17:34 > 0:17:39Both forces dug tunnels into the mountain to avoid enemy shells.
0:17:40 > 0:17:47They dragged huge guns into fortified positions at heights of up to 3,500 metres.
0:17:47 > 0:17:51These guns dominated entire valleys, making an attack
0:17:51 > 0:17:55up the steep, rugged slopes dangerous and frightening.
0:17:57 > 0:18:02But the most terrifying enemy was nature itself.
0:18:02 > 0:18:07In just one night in December 1916, after days of heavy snowfall,
0:18:07 > 0:18:12105 avalanches thundered across the battle lines.
0:18:22 > 0:18:2610,000 men died in this night of horror.
0:18:32 > 0:18:38But these colossal casualties actually gave the generals a new and deadly tactic.
0:18:38 > 0:18:43Both sides realised that they could turn avalanches into weapons
0:18:43 > 0:18:48by deliberately triggering them to strike enemy lines and bury the soldiers alive.
0:18:55 > 0:18:58So the guns were moved and pointed in a new direction.
0:18:58 > 0:19:05They were no longer aimed directly at the enemy, but at the ridges of snow above them.
0:19:05 > 0:19:10A single round could dislodge an entire Alpine snowfield,
0:19:10 > 0:19:14sending an avalanche hurtling down the mountainside, onto the enemy below.
0:19:31 > 0:19:38Entire platoons were killed, without a single shot being fired in return.
0:19:38 > 0:19:43In the battle for the Alpine front, the power of nature proved to be
0:19:43 > 0:19:47a far more potent weapon than anything man could invent.
0:19:59 > 0:20:03Even today, the remains of soldiers who fought in the Alps during World War I
0:20:03 > 0:20:09are still being discovered as the glaciers on the mountain retreat.
0:20:09 > 0:20:16These two Austrian soldiers were dug from the ice in 2004, still in their uniform.
0:20:21 > 0:20:25In the end, whether natural or man-made,
0:20:25 > 0:20:29avalanches claimed the lives of thousands of soldiers.
0:20:29 > 0:20:34To use avalanches as a weapon of war was a brutal tactic,
0:20:34 > 0:20:38but then again, it was a brutal war.
0:20:51 > 0:20:55It's not just harsh winters that cause avalanches -
0:20:55 > 0:20:59it looks like global warming could be triggering them as well.
0:20:59 > 0:21:03As the climate changes, snow and ice on the mountains,
0:21:03 > 0:21:08and glaciers like that over there, become increasingly unpredictable.
0:21:08 > 0:21:16Some believe that global warming contributed to the collapse of the Kolka glacier in Russia in 2002.
0:21:16 > 0:21:21The collapse produced a huge avalanche that swept away an entire village.
0:21:21 > 0:21:25It was the worst avalanche disaster in Russian history.
0:21:31 > 0:21:34A glacier is a slow moving river of ice,
0:21:34 > 0:21:39formed by compacted snow laid down over thousands of years.
0:21:45 > 0:21:49Once it reaches a critical thickness, around 10 metres,
0:21:49 > 0:21:52the ice becomes so heavy that it begins to move,
0:21:52 > 0:21:54pulled down by the force of gravity.
0:21:56 > 0:22:02Glaciers are known to advance and retreat, but usually only a matter of metres a year.
0:22:02 > 0:22:05But in the Caucasus Mountains in Russia,
0:22:05 > 0:22:12an avalanche triggered the Kolka glacier to move on a different scale altogether.
0:22:19 > 0:22:26On the night of September 20th 2002, a wall of ice collapsed from the steep mountain above.
0:22:26 > 0:22:29It created a small, but powerful avalanche.
0:22:29 > 0:22:33This avalanche smashed into the Kolka glacier.
0:22:33 > 0:22:37Seismometers, usually used to record the size of earthquakes,
0:22:37 > 0:22:39registered the impact.
0:22:40 > 0:22:47The impact of the avalanche hitting the glacier was so great the unimaginable happened.
0:22:47 > 0:22:51Incredibly, the entire Kolka glacier broke free.
0:22:56 > 0:22:59While it normally moved at a snail's pace,
0:22:59 > 0:23:04now the entire glacier raced down the mountain at over 100 miles an hour.
0:23:10 > 0:23:14Millions of tons of ice and rock surged towards the settlements below.
0:23:14 > 0:23:20Just five minutes and 15 miles later, it reached the village of Karmadon.
0:23:32 > 0:23:37Within seconds, the village was engulfed in ice, rock and mud.
0:23:37 > 0:23:40No-one in its path stood a chance.
0:23:40 > 0:23:43Over 120 people died.
0:23:51 > 0:23:57This was a unique geological event and the largest ice avalanche ever documented.
0:24:09 > 0:24:12Scientists now believe that the first small avalanche,
0:24:12 > 0:24:15which happened high above the Kolka glacier
0:24:15 > 0:24:18may have melted and weakened due to global warming.
0:24:18 > 0:24:22And this kind of avalanche may happen more frequently
0:24:22 > 0:24:26as the earth's atmospheric temperature increases.
0:24:28 > 0:24:33But that doesn't explain the extraordinary collapse of the entire Kolka glacier.
0:24:33 > 0:24:36Some scientists now think that volcanic activity
0:24:36 > 0:24:39may have warmed the ground beneath the glacier,
0:24:39 > 0:24:42softening the bottom layer of ice;
0:24:42 > 0:24:44so when an avalanche smashed into it,
0:24:44 > 0:24:48the glacier was already primed for destruction.
0:24:50 > 0:24:55What we do know is that global warming makes snow and ice in mountain regions
0:24:55 > 0:24:58far more unstable and less predictable,
0:24:58 > 0:25:04and this might well create more avalanches in the future than we ever would have expected.
0:25:04 > 0:25:08My next avalanche is a story seldom told,
0:25:08 > 0:25:11but it's the worst avalanche that ever hit America.
0:25:11 > 0:25:18The town that it struck no longer exists because it never recovered from the calamity that befell it.
0:25:18 > 0:25:22What occurred in the Cascade Mountains in 1910
0:25:22 > 0:25:26was a far greater tragedy than it should ever have been,
0:25:26 > 0:25:29because of one terrible coincidence.
0:25:29 > 0:25:32The avalanche happened to strike the exact spot
0:25:32 > 0:25:35where two trains were trapped in the snow.
0:25:53 > 0:25:56The Great Northern Railway in the United States
0:25:56 > 0:26:00passes through the Cascade Mountains in Washington State.
0:26:01 > 0:26:04There was once a small town here called Wellington.
0:26:04 > 0:26:08They were used to freezing weather here
0:26:08 > 0:26:11and were well-equipped to deal with it.
0:26:11 > 0:26:14Tunnels kept the worst of the snow off the tracks,
0:26:14 > 0:26:17and they had ploughs to clear the snow that remained.
0:26:22 > 0:26:26But these defences were powerless in the face of a terrible blizzard
0:26:26 > 0:26:30that struck the area in February 1910.
0:26:30 > 0:26:37For nine whole days it raged, with a foot of snow falling each hour.
0:26:40 > 0:26:45The snowfall was so heavy that the track was closed,
0:26:45 > 0:26:50leaving two trains bound for Seattle stuck fast outside Wellington;
0:26:50 > 0:26:53a passenger train and a mail train.
0:26:56 > 0:27:01For five days, the trains were trapped by 25ft drifts.
0:27:01 > 0:27:05High above on Windy Mountain, the snow piled up.
0:27:07 > 0:27:09The only relief for the passengers and crew
0:27:09 > 0:27:14was to trek into Wellington for something to eat and to buy tobacco.
0:27:14 > 0:27:19But then they returned to the carriages to sleep every night.
0:27:24 > 0:27:28What bad luck, then, that the avalanche struck at night.
0:27:34 > 0:27:40Shortly after midnight on March 1st 1910 came a violent thunderstorm.
0:27:43 > 0:27:46Dislodged by the storm, or by its own mass,
0:27:46 > 0:27:48a huge swathe of cement-like snow
0:27:48 > 0:27:52now poured down the mountain towards the carriages below.
0:28:05 > 0:28:11A 10 foot wall of snow, half a mile wide, raced towards the trains.
0:28:13 > 0:28:18The passengers fast asleep in the carriages had no idea what was coming.
0:28:29 > 0:28:36They were swept violently into the air and thrown 150 feet down into the valley below.
0:28:39 > 0:28:43Steam hissed from the wreckage of the train.
0:28:43 > 0:28:46The snow was red with blood.
0:28:46 > 0:28:5396 men, women and children died, in America's worst avalanche disaster.
0:28:54 > 0:28:58Astonishingly, 23 passengers were rescued alive.
0:28:59 > 0:29:02But when every survivor had been pulled from the wreckage,
0:29:02 > 0:29:09the grim task of collecting the bodies was abandoned because the conditions were so bad.
0:29:11 > 0:29:16It wasn't until the following summer that the last of the dead were finally retrieved.
0:29:22 > 0:29:29Surprisingly, it may have been the rain that caused this catastrophic avalanche.
0:29:29 > 0:29:31Wet snow has less strength than dry snow.
0:29:31 > 0:29:34Water weakens the bonds between the ice crystals
0:29:34 > 0:29:38and the added weight reduces the stability of the snow pack.
0:29:43 > 0:29:46Warm air from the Pacific had travelled to the Cascade Mountains,
0:29:46 > 0:29:50creating conditions for a thunder and lightning storm.
0:29:50 > 0:29:54And the warmer temperature, together with the rain that followed,
0:29:54 > 0:29:58would have loosened the snow and made it more susceptible to collapse.
0:30:03 > 0:30:07All these factors almost certainly combined
0:30:07 > 0:30:12to dislodge a vast slab of snow on that fateful night in Wellington -
0:30:12 > 0:30:15to this day, America's worst avalanche disaster.
0:30:23 > 0:30:27My next story takes us back to the winter of 1951,
0:30:27 > 0:30:31when Europe was experiencing some of its harshest weather.
0:30:31 > 0:30:34The Alps were struck by thousands of avalanches.
0:30:34 > 0:30:40It was the worst winter in recorded history and dubbed the Winter of Terror.
0:30:40 > 0:30:47But, in a way, out of that terror came hope because things got so bad in 1951 that all the Alpine nations
0:30:47 > 0:30:53came together to do something about it, and we are still reaping the safety benefits from that today.
0:30:59 > 0:31:06No-one was prepared for the scale of misery that descended on the Alps during the Winter of Terror in 1951.
0:31:07 > 0:31:14Over the course of just a few months, thousands of avalanches struck, killing over 200 people.
0:31:18 > 0:31:23Roads were blocked, leaving communities stranded without supplies
0:31:23 > 0:31:27and unable to escape the danger zones.
0:31:27 > 0:31:34The relentless tragedies meant that when an avalanche struck, everyone would help with the rescue efforts.
0:31:36 > 0:31:41It soon became clear that it was impossible to get to everyone in time.
0:31:47 > 0:31:49But this was also a turning point.
0:31:49 > 0:31:53Things got so bad that it prompted all the nations of the Alps
0:31:53 > 0:31:59to dramatically increase the measures taken to protect their citizens.
0:31:59 > 0:32:06They turned to the first dedicated Avalanche Research Institute at Davos, Switzerland,
0:32:06 > 0:32:13who began to focus their attention on devising new ways of protecting people from avalanches.
0:32:13 > 0:32:16Having studied the physics and mechanics of snow,
0:32:16 > 0:32:24their knowledge was now applied to the causes of avalanches - how to predict them and prevent them.
0:32:24 > 0:32:29They examined snow crystals, working out how they affect the stability of the snow pack,
0:32:29 > 0:32:32and they studied weak layers under the microscope
0:32:32 > 0:32:37to see how they are influenced by different weather conditions.
0:32:37 > 0:32:41We're still reaping the benefits today from what they learned.
0:32:45 > 0:32:49The Winter of Terror led to the creation of new kinds of avalanche defences -
0:32:49 > 0:32:52rows of fences to hold back the snow,
0:32:52 > 0:32:58as well as rescue services and avalanche zoning to show safe and dangerous areas.
0:33:00 > 0:33:05But there was another crucial thing that came out of the Winter of Terror -
0:33:05 > 0:33:11it was now that people took up the practice of starting avalanches deliberately.
0:33:18 > 0:33:21Not to kill, but to save lives.
0:33:21 > 0:33:28And this has become the single most important part of avalanche safety we have.
0:33:29 > 0:33:34There is now an army of about 2,000 men and women
0:33:34 > 0:33:39who deliberately start avalanches in the most dangerous places in the Alps.
0:33:39 > 0:33:45They're adrenaline junkies. They like to blow things up, and their fix is blasting mountain slopes.
0:33:45 > 0:33:52And the government pays them sometimes as little as just £15 a day danger money to do it.
0:33:55 > 0:33:58For that pay, I guess you just have to love the job!
0:34:01 > 0:34:07Any slope that looks like it might be about to cause an avalanche is cleared of skiers,
0:34:07 > 0:34:13then these intrepid snow blasters turn up - often by helicopter - to cause a controlled avalanche
0:34:13 > 0:34:20that clears the snow safely so that large slab avalanches don't develop and threaten the slopes below.
0:34:26 > 0:34:30Of course, it's not safe for them.
0:34:30 > 0:34:33This highly skilled job is extremely dangerous.
0:34:36 > 0:34:41Their tool kit consists of rope, fuse and dynamite.
0:34:45 > 0:34:52Dynamite is attached to a cord to prevent it sliding too far down the slope and causing danger below.
0:34:52 > 0:34:59If the dynamite doesn't detonate the first time, they've no option but to reel it in and try again.
0:35:01 > 0:35:08But their greatest challenge lies in knowing how to get away from the avalanche they've created.
0:35:08 > 0:35:10That's quite a skill.
0:35:12 > 0:35:19Thanks to people like this who risk their lives for us, people can ski here more safely,
0:35:19 > 0:35:26and all because some human snow-blasters are prepared to clear the slopes with dynamite.
0:35:36 > 0:35:40My next avalanche happened in the Andes of South America,
0:35:40 > 0:35:45and it led to one of the most bizarre aviation mysteries of all time.
0:35:45 > 0:35:49The avalanche caused an entire aeroplane to vanish.
0:35:49 > 0:35:56It mysteriously reappeared 50 years later, complete with its passengers and crew.
0:36:00 > 0:36:06This story starts just after the Second World War, before the age of commercial jet travel,
0:36:06 > 0:36:13at a time when the planes that passengers travelled in remind me of scenes from the movie Casablanca.
0:36:22 > 0:36:28On August 2nd 1947, a converted Lancaster bomber, the Stardust,
0:36:28 > 0:36:32took off on a regular passenger flight across South America.
0:36:32 > 0:36:37But the flight itself proved to be anything but routine.
0:36:37 > 0:36:41It should have taken four hours to travel from Buenos Aires in Argentina
0:36:41 > 0:36:46to the Chilean capital Santiago across the Andes Mountains.
0:36:46 > 0:36:50At the controls was a highly experienced pilot.
0:36:53 > 0:36:56Some of the six passengers on board seemed to have stepped straight out
0:36:56 > 0:37:03of an Agatha Christie novel - a Palestinian businessman with a large diamond sewn into his jacket,
0:37:03 > 0:37:08a German emigre returning to Chile with the ashes of her dead husband,
0:37:08 > 0:37:14and a British King's messenger, apparently carrying vital diplomatic correspondence.
0:37:14 > 0:37:18But no-one on board was ever to reach their destination.
0:37:22 > 0:37:28Just before Stardust was due to land, the plane sent a mysterious Morse Code message -
0:37:28 > 0:37:30S-T-E-N-D-E-C.
0:37:35 > 0:37:41Baffled by this unintelligible word, the radio operator in Santiago asked for clarification.
0:37:41 > 0:37:49The same word - S-T-E-N-D-E-C - was repeated twice more and, after that, nothing more was heard.
0:37:53 > 0:37:56The plane simply vanished.
0:37:59 > 0:38:05The search began, but there was no sign of Stardust around Santiago,
0:38:05 > 0:38:09so the search spread out to cover the Andes mountains.
0:38:09 > 0:38:14Aeroplanes criss-crossed a vast area, but the searchers found nothing.
0:38:14 > 0:38:18The plane had completely disappeared.
0:38:18 > 0:38:23And so began one of aviation's most enduring mysteries.
0:38:23 > 0:38:25Rumours were rife -
0:38:25 > 0:38:30the plane had been blown up because of the documents carried by the King's messenger,
0:38:30 > 0:38:36alien abduction was suggested, and just what did S-T-E-N-D-E-C mean?
0:38:36 > 0:38:41What we would eventually find out was even more surprising.
0:38:41 > 0:38:47In the year 2000, a mysterious discovery reopened the case of the vanishing plane.
0:38:47 > 0:38:54An old Rolls Royce engine was found lying on a glacier high in the Andes mountains.
0:38:56 > 0:39:00It had appeared out of nowhere, and it belonged to Stardust.
0:39:02 > 0:39:04Over the last 50 years,
0:39:04 > 0:39:09the area had been frequently visited by mountaineers and they'd found nothing.
0:39:09 > 0:39:16So why had the wreckage, together with human remains, suddenly appeared out of nowhere?
0:39:16 > 0:39:23It took the combined efforts of an air crash investigator and a glaciologist to solve the mystery
0:39:23 > 0:39:27and, believe me, it's better than any Agatha Christie story.
0:39:31 > 0:39:34All those years ago, in 1947,
0:39:34 > 0:39:41Stardust had been flying over the mountains in bad weather and strayed hopelessly off course.
0:39:41 > 0:39:46Miscalculating their position, the pilot thought they were near Santiago
0:39:46 > 0:39:48and began their descent to land.
0:39:48 > 0:39:55But it wasn't the runway ahead - it was Mount Tupungato, one of the highest mountains in the Andes,
0:39:55 > 0:39:58and they crashed straight into it.
0:40:03 > 0:40:10The impact of the crash set off a massive avalanche that completely covered the plane.
0:40:12 > 0:40:18The wreckage was now totally hidden from the view of aircraft looking for the crash site.
0:40:24 > 0:40:26Stardust disappeared under the avalanche,
0:40:26 > 0:40:33but what made the plane vanish for 50 years was that it was swallowed up by the glacier.
0:40:35 > 0:40:39So how exactly can a glacier swallow a plane?
0:40:40 > 0:40:48Year by year, layers of snow would have buried the wreckage deeper and deeper inside the glacier.
0:40:48 > 0:40:52Gradually, Stardust would become part of the glacier itself,
0:40:52 > 0:40:58travelling slowly downhill - not on the surface of the ice, but deep inside.
0:41:00 > 0:41:04It would move no more than a few metres a year
0:41:04 > 0:41:11until, 53 years later in 2000, the plane and its passengers emerged at the bottom of the glacier.
0:41:12 > 0:41:19The avalanche that covered the plane had merely started this extraordinary vanishing act.
0:41:22 > 0:41:26So the mystery was solved. Well, kind of.
0:41:26 > 0:41:33To this day, no-one understands what was meant by the final Morse Code message - S-T-E-N-D-E-C.
0:41:33 > 0:41:39So a piece of the puzzle remains missing about the story of Stardust.
0:41:50 > 0:41:54My next story is about an avalanche that happened in Austria.
0:41:54 > 0:41:59This wasn't only a human tragedy, it was also a scientific mystery -
0:41:59 > 0:42:05a mystery that was only finally solved in one of the most unusual avalanche experiments ever.
0:42:05 > 0:42:11Somehow scientists managed to put themselves inside an avalanche whilst it was happening.
0:42:11 > 0:42:16They all lived to tell the tale, and the evidence that they gathered
0:42:16 > 0:42:19revealed something completely new about avalanches.
0:42:29 > 0:42:35Galtur is a small village in Austria, close to the Swiss border.
0:42:35 > 0:42:39Like many other towns in the Alps, this was designated a safe area,
0:42:39 > 0:42:44built far enough away from the base of the mountain that no avalanche should ever reach it.
0:42:46 > 0:42:49In the past, a small avalanche followed the same route each year,
0:42:49 > 0:42:53but trickled out safely at the bottom of the slope.
0:42:54 > 0:43:00Computer models had indicated that no normal avalanche would have the power to reach the village,
0:43:00 > 0:43:04over 200 metres from the foot of the mountain.
0:43:04 > 0:43:11And yet, like too many apparently safe towns, the unexpected happened here in 1999
0:43:11 > 0:43:15when it was hit by a massive avalanche.
0:43:15 > 0:43:20In only 50 seconds, the avalanche raced down the mountain
0:43:20 > 0:43:24and crossed a flat valley that should have stopped it dead.
0:43:24 > 0:43:26But this avalanche didn't stop.
0:43:26 > 0:43:33It kept on going across the valley and hit the village at a speed of almost 200 mph.
0:43:50 > 0:43:5331 people died.
0:43:53 > 0:43:58It was described by survivors as being like a huge Tsunami.
0:44:00 > 0:44:04This is not the first time an avalanche has defied expectations
0:44:04 > 0:44:08and struck in a place no avalanche should reach.
0:44:08 > 0:44:12Scientists had been determined to understand why this kept happening.
0:44:12 > 0:44:17What was missing from their computer models, from their understanding of avalanches,
0:44:17 > 0:44:23that meant they were underestimating the potential power of certain avalanches?
0:44:25 > 0:44:32The scientists at Davos had studied the physics of snow for decades under laboratory conditions,
0:44:32 > 0:44:38but they realised that there must be something more going on inside an avalanche that they had never seen.
0:44:45 > 0:44:48They were determined to work out what it was.
0:44:50 > 0:44:55But instead of bringing more snow to the lab, they took the lab to the snow,
0:44:55 > 0:45:01placing their instruments and themselves inside an avalanche.
0:45:01 > 0:45:08But how could they get right inside the lethal onslaught of an avalanche while it was in full flow?
0:45:13 > 0:45:16They came up with an ingenious solution.
0:45:18 > 0:45:25They built a reinforced concrete bunker smack in the middle of avalanche territory.
0:45:30 > 0:45:35Then, in February 1999, the scientists put themselves
0:45:35 > 0:45:42and a mass of scientific equipment including radar, pressure monitors and flow measures into the bunker,
0:45:42 > 0:45:45locked the heavy iron doors behind them
0:45:45 > 0:45:48and waited.
0:45:48 > 0:45:55Meanwhile, far above them, a massive avalanche was deliberately set off with explosives.
0:46:01 > 0:46:04It was directed straight at them.
0:46:05 > 0:46:10As snow thundered down the mountain, the bunker was completely engulfed.
0:46:12 > 0:46:15They hadn't anticipated quite how big the avalanche would be.
0:46:24 > 0:46:28We were surprised by the force by which the avalanche hit the shelter.
0:46:28 > 0:46:31We did not expect such a big thing to come down.
0:46:31 > 0:46:36This is a very massive and solid bunker - I think the walls are about 40 centimetres thick -
0:46:36 > 0:46:42and you could feel the vibrations of the whole building.
0:46:46 > 0:46:48We heard a strong noise.
0:46:48 > 0:46:52That was because the door broke open and the snow came in.
0:46:52 > 0:46:54Then the pressure in the shelter rose enormously.
0:46:54 > 0:46:57It was like diving into two metres of water.
0:47:00 > 0:47:05After the avalanche had hit us, we had first to try to get out,
0:47:05 > 0:47:08because the shelter was completely covered by snow.
0:47:08 > 0:47:12And so we started digging a tunnel out, and then we had to work hard.
0:47:12 > 0:47:14It was, really, very compact snow.
0:47:17 > 0:47:21So, after all this, what had they discovered?
0:47:21 > 0:47:27Radar allowed the scientists to peer inside each layer of snow as the avalanche was happening.
0:47:29 > 0:47:35This revealed a critical and deadly layer of snow that no-one had understood before.
0:47:37 > 0:47:45It's called the saltation layer - a band of dense, heavy snow in the middle of the avalanche flow.
0:47:45 > 0:47:50Scientists discovered that this saltation layer is what can turbo-charge an avalanche,
0:47:50 > 0:47:53boosting its power and speed.
0:47:53 > 0:47:59As the heavy snow particles in the saltation layer bounce around within the avalanche,
0:47:59 > 0:48:04they gather up new snow, dramatically increasingly its volume and power
0:48:04 > 0:48:10so that the whole mass plummets down a slope with unstoppable force.
0:48:12 > 0:48:15Based on this research, these scientists were now able
0:48:15 > 0:48:20to explain how the Galtur avalanche reached the town.
0:48:20 > 0:48:23It was the saltation layer that allowed the avalanche
0:48:23 > 0:48:29to build up energy and momentum and so travel across the flat valley floor and destroy the village.
0:48:31 > 0:48:37Thanks to the discovery in the bunker, today the equations for the saltation layer
0:48:37 > 0:48:42are routinely added to computer models to assess avalanche risks throughout the world.
0:48:45 > 0:48:50Since Galtur, the bunker experiment has been performed many times,
0:48:50 > 0:48:54and each time it reveals something new about avalanches.
0:48:54 > 0:48:59Scientists have come to realise that in order to really understand how nature works,
0:48:59 > 0:49:03they've got to be prepared to put themselves in the line of fire.
0:49:10 > 0:49:14Our next avalanche is the worst in history.
0:49:14 > 0:49:20It happened in Peru and it killed more people than any other avalanche we know of.
0:49:20 > 0:49:25It was enormous, and that's because it was triggered by a massive earthquake.
0:49:33 > 0:49:38South America boasts some of the world's most stunning landscapes,
0:49:38 > 0:49:45but its unstable geology also makes it an unpredictable and deadly area.
0:49:45 > 0:49:47Earthquakes and volcanoes strike without warning.
0:49:52 > 0:49:58May 31st 1970, the opening day of the football World Cup.
0:49:58 > 0:50:04Soccer is Peru's national sport and because the country had qualified,
0:50:04 > 0:50:09people were out in the streets of virtually every town and village, having a fiesta.
0:50:15 > 0:50:20But while the celebrations were in full swing, disaster was waiting in the wings.
0:50:22 > 0:50:29Deep in the Pacific ocean, 20 miles off Peru's coast, the seafloor ruptured.
0:50:29 > 0:50:34A massive earthquake sent shock waves racing towards Peru's mainland.
0:50:38 > 0:50:42Within seconds, the first shock waves hit the coast,
0:50:42 > 0:50:46and soon reached the snow-capped peaks of the Peruvian Andes.
0:50:48 > 0:50:52As the shock waves hit Mount Huascaran, the highest mountain in Peru,
0:50:52 > 0:50:56it triggered an enormous glacial avalanche.
0:50:56 > 0:51:01What is so significant about this avalanche is its sheer size.
0:51:01 > 0:51:06This one was a mile long and half a mile wide.
0:51:06 > 0:51:08Travelling at over 100 miles per hour,
0:51:08 > 0:51:13it launched the most lethal and destructive avalanche the world has ever seen.
0:51:17 > 0:51:24It raced down the 20,000 foot mountain, picking up rocks and debris as it went.
0:51:24 > 0:51:28It took just a few minutes for the deadly mass of snow, water,
0:51:28 > 0:51:34mud and rock to travel ten miles, all the way to the town of Yungay.
0:51:37 > 0:51:42It engulfed the entire settlement, covering it in mud many metres deep.
0:51:46 > 0:51:51It's believed that 18,000 people perished in the town that day.
0:51:53 > 0:51:56Yungay became a ghostly place.
0:51:58 > 0:52:03The tops of four palm trees were all that remained of its central square.
0:52:06 > 0:52:10No-one knows exactly how many bodies are buried here.
0:52:10 > 0:52:17The Peruvian government has forbidden the excavation of Yungay, declaring it a national cemetery,
0:52:17 > 0:52:20and so much of the area remains eerily as it was
0:52:20 > 0:52:25when the most deadly avalanche in history wiped out this town.
0:52:32 > 0:52:37My final avalanche is the most terrifying monster of all.
0:52:37 > 0:52:41This particular kind of avalanche has rarely been filmed or photographed,
0:52:41 > 0:52:44because anyone who sees them is usually killed by them.
0:52:44 > 0:52:48They're are the most deadly avalanche of all -
0:52:48 > 0:52:49rock avalanches.
0:52:53 > 0:52:57We usually think of avalanches as tumbling snow,
0:52:57 > 0:53:02but actually anything that flows at high speed is called an avalanche.
0:53:02 > 0:53:06And what's amazing is that under certain rare conditions,
0:53:06 > 0:53:12even rock can behave like snow, cascading down a mountainside just like a snow avalanche.
0:53:13 > 0:53:16I'm not talking about a landslide.
0:53:16 > 0:53:23This is a landslide - a slow-moving mass of rock and earth pulled down by gravity.
0:53:27 > 0:53:29But just take a look at this.
0:53:36 > 0:53:41Although it looks like a river of water, it's just rock and dust.
0:53:48 > 0:53:54As huge pieces of rock break free from the mountain and cascade down a slope,
0:53:54 > 0:53:58the rocks break up into smaller and smaller pieces.
0:54:01 > 0:54:05Fine dust particles change the dynamics of the flow,
0:54:05 > 0:54:10the physics of which are extremely complex, just like snow avalanches.
0:54:13 > 0:54:20This is a controlled experiment by scientists - trying to film the real thing happening is too dangerous.
0:54:20 > 0:54:27In these experiments, scientists use rock debris to demonstrate how rock behaves in an avalanche.
0:54:31 > 0:54:36Exactly how rocks able to flow and move so far is still not clear.
0:54:36 > 0:54:41Some scientists suggest that the avalanche flows on a bed of air,
0:54:41 > 0:54:48others say that when the rocks hit the valley bottom they explode, releasing stored energy.
0:54:48 > 0:54:51The simple answer is we still don't know.
0:54:54 > 0:55:00Rock avalanches are very rare, and it's even rarer to actually witness one.
0:55:00 > 0:55:03That's why scientists still know so little about them.
0:55:03 > 0:55:06But we do know that it is this flowing property
0:55:06 > 0:55:11that means the rocks don't stop when they reach the valley floor.
0:55:11 > 0:55:16They flow out over the surrounding area, just like a snow avalanche,
0:55:16 > 0:55:20threatening towns and villages far away from the slope.
0:55:23 > 0:55:26That's what happened to the town of Frank in Canada.
0:55:28 > 0:55:35In 1903, it was hit by a rock avalanche which struck without warning.
0:55:40 > 0:55:4490 million tonnes of rock fell from the summit of Turtle Mountain,
0:55:44 > 0:55:51and when it hit the ground, it spread over a two mile area of the valley in less than two minutes.
0:55:59 > 0:56:05Survivors in the town of Frank recalled hearing a terrible sound
0:56:05 > 0:56:10like cannon fire echoing around the valley as the mountain collapsed.
0:56:12 > 0:56:1670 of the 600 inhabitants were killed.
0:56:16 > 0:56:20Most of the bodies were never recovered -
0:56:20 > 0:56:23they were buried beneath the rubble.
0:56:25 > 0:56:32Despite their unpredictability, we can expect more of these terrifying rock avalanches in the future.
0:56:34 > 0:56:42Scientists can now identify the danger zones because they've studied unstable slopes around the world.
0:56:42 > 0:56:45What we know is that, although they'll always be rare,
0:56:45 > 0:56:52these extraordinary rock avalanches will happen again - with lethal consequences.
0:57:02 > 0:57:08So we come to the end of our ten remarkable stories about avalanches.
0:57:10 > 0:57:15If we choose to live and work in the shadow of mountains like these,
0:57:15 > 0:57:18we're always gonna be at risk from avalanches.
0:57:18 > 0:57:24No matter how safe things might appear and despite all the advances of modern science,
0:57:24 > 0:57:27nature will always have the last word.