0:00:02 > 0:00:03It's a tornado!
0:00:03 > 0:00:05Look at that!
0:00:11 > 0:00:16Our planet is home to some spectacular natural wonders.
0:00:19 > 0:00:25Yet exactly how and why they form is still a mystery.
0:00:28 > 0:00:31But now new camera technologies
0:00:31 > 0:00:34are revealing their inner workings in stunning detail.
0:00:38 > 0:00:41My name is Dr Helen Czerski
0:00:41 > 0:00:45and I'll be looking at how these extraordinary images
0:00:45 > 0:00:50are transforming our understanding of the natural world.
0:00:54 > 0:00:58In this programme, we uncover the latest scientific insights
0:00:58 > 0:01:01into the devastating power of avalanches.
0:01:04 > 0:01:08The scale and grandeur of an avalanche are gigantic
0:01:08 > 0:01:11and yet, many of the details needed to understand them
0:01:11 > 0:01:13lie in the world of the really tiny.
0:01:16 > 0:01:20Now, detailed CT scans are showing how microscopic changes in snow
0:01:20 > 0:01:23can cause an avalanche at the lightest touch.
0:01:24 > 0:01:26The latest computer models
0:01:26 > 0:01:31are revealing why the 2015 Everest avalanche was so deadly.
0:01:33 > 0:01:37And extraordinary eyewitness footage is giving vital clues
0:01:37 > 0:01:41about how avalanche snow can seize up like concrete.
0:01:41 > 0:01:44It's new findings like these
0:01:44 > 0:01:48that allow scientists to peer deeper inside the anatomy of an avalanche
0:01:48 > 0:01:49than ever before.
0:02:09 > 0:02:14Snow draws millions into the mountains each winter,
0:02:14 > 0:02:16but snow can also be deadly
0:02:16 > 0:02:20and avalanche scientists are trying to understand why.
0:02:24 > 0:02:29For ski guide Kristoffer Carlsson, the morning of February 28th 2011
0:02:29 > 0:02:31started like any other.
0:02:32 > 0:02:37This day we woke up really early, I think at 6:30am in the morning,
0:02:37 > 0:02:40because we had seen on the weather forecast
0:02:40 > 0:02:42that it was supposed to be a really beautiful day
0:02:42 > 0:02:46and the past three or four days it had been snowing quite heavily.
0:02:46 > 0:02:49But on this particular day
0:02:49 > 0:02:52we were just so happy about the sun being out again
0:02:52 > 0:02:54and we were just looking forward
0:02:54 > 0:02:56to one of the greatest ski days of the season.
0:02:56 > 0:03:01TRANSLATION FROM GERMAN:
0:03:04 > 0:03:06The day was so perfect,
0:03:06 > 0:03:09Kristoffer decided to record everything on his helmet camera.
0:03:11 > 0:03:14His footage reveals one of the strange properties of snow
0:03:14 > 0:03:16that makes avalanches so deadly.
0:03:18 > 0:03:21My friends, they went down on the left side
0:03:21 > 0:03:25and I chose going a bit more to the right.
0:03:28 > 0:03:33I saw that there was a lot of fresh snow on that route
0:03:33 > 0:03:38and then everything just happened in a second.
0:03:46 > 0:03:48Kristoffer triggered an avalanche
0:03:48 > 0:03:51that carried him 200 metres down the mountain.
0:03:54 > 0:03:58There was a huge amount of snow taking my skis away.
0:03:59 > 0:04:02It felt like being in a washing machine,
0:04:02 > 0:04:04just tumbling down the mountain.
0:04:06 > 0:04:10He knew he had to try and stay on top of the snow at all costs.
0:04:12 > 0:04:17The only thought I had was, "Don't get buried, don't get buried,"
0:04:17 > 0:04:22because, if you get buried, your chances drop drastically.
0:04:38 > 0:04:41Kristoffer ended up buried under two metres of snow,
0:04:41 > 0:04:44feet pointing upwards and unable to move.
0:04:44 > 0:04:47HE GROANS AND STRAINS
0:04:47 > 0:04:51The moment that I realised that I was completely buried,
0:04:51 > 0:04:57I remember that I was quite shocked about how hard the snow was.
0:04:57 > 0:05:02It really turned into something that felt like concrete
0:05:02 > 0:05:05in an instant, really.
0:05:12 > 0:05:14HE GROANS
0:05:14 > 0:05:16I couldn't move.
0:05:16 > 0:05:19HE CRIES OUT
0:05:19 > 0:05:24The pressure of the snow on my chest made it really hard to breathe.
0:05:24 > 0:05:25HE CRIES OUT
0:05:27 > 0:05:31The only thing that I could do was trying to stay calm.
0:05:31 > 0:05:34HE CRIES OUT
0:05:41 > 0:05:45As if entombed in concrete, Kristoffer couldn't move.
0:05:45 > 0:05:47HE CRIES OUT
0:05:47 > 0:05:51And he only had a small pocket of air around his mouth to breathe.
0:05:54 > 0:05:56HE CRIES OUT
0:05:59 > 0:06:01HE CRIES OUT
0:06:03 > 0:06:04VOICES
0:06:07 > 0:06:10SHOUTING
0:06:13 > 0:06:15It took a terrifying five minutes
0:06:15 > 0:06:18until his friends managed to dig him out.
0:06:19 > 0:06:21This is so amazing.
0:06:22 > 0:06:25Kristoffer was incredibly lucky.
0:06:25 > 0:06:29Only half of all people buried in an avalanche like this survive.
0:06:31 > 0:06:33It's impossible to dig yourself out
0:06:33 > 0:06:36and Kristoffer's footage reveals why.
0:06:38 > 0:06:41Kristoffer describes the snow setting like concrete
0:06:41 > 0:06:43when it stops moving. And that's weird.
0:06:43 > 0:06:46How does something as light and fluffy as snow become a solid?
0:06:46 > 0:06:49Well, there's a clue in the footage.
0:06:49 > 0:06:53As he starts to fall and the snow comes rushing past him,
0:06:53 > 0:06:56the snow grains are rushing over each other, bumping into each other,
0:06:56 > 0:07:00and there's lots of friction and that is generating heat.
0:07:00 > 0:07:03So the outside of the snow grains are starting to melt slightly.
0:07:06 > 0:07:09And once you get this thin layer of water around a snow grain,
0:07:09 > 0:07:12it behaves in a peculiar way.
0:07:14 > 0:07:15These ice cubes
0:07:15 > 0:07:17behave like snow grains in the avalanche once it's stopped.
0:07:17 > 0:07:20They're really close to their melting point,
0:07:20 > 0:07:22so their surface is like a thin layer of water.
0:07:22 > 0:07:24Those molecules are really mobile.
0:07:24 > 0:07:27And if I push two of them together
0:07:27 > 0:07:29and then take my hand away, they stick.
0:07:29 > 0:07:31And what happened was that thin layer of water,
0:07:31 > 0:07:34when it was stuck between the two ice cubes,
0:07:34 > 0:07:36the ice cubes stole its heat away
0:07:36 > 0:07:39and so it re-froze, gluing them together.
0:07:39 > 0:07:42This is called ice sintering
0:07:42 > 0:07:45and this is what happens to the snow grains in the avalanche
0:07:45 > 0:07:47and it's what makes the snow pack go solid.
0:07:48 > 0:07:53And it's because of ice sintering that Kristoffer was unable to move,
0:07:53 > 0:07:54let alone dig himself out,
0:07:54 > 0:07:57as he was buried deep under the snow.
0:08:08 > 0:08:10More than a million avalanches
0:08:10 > 0:08:13happen throughout the world every year.
0:08:16 > 0:08:21In an average winter, about 500 people die in avalanches.
0:08:22 > 0:08:27The largest can destroy whole towns and kill thousands.
0:08:30 > 0:08:33So understanding them is crucial
0:08:33 > 0:08:36for protecting people's lives and livelihoods.
0:08:43 > 0:08:48What's surprising about avalanches is that all their destructive force
0:08:48 > 0:08:51comes from simple snowflakes
0:08:51 > 0:08:54and the way they change at a microscopic level.
0:08:57 > 0:09:02All snowflakes start off in the heart of frozen clouds.
0:09:02 > 0:09:09They begin life as an ice crystal, a six-sided shape a bit like this.
0:09:13 > 0:09:18As water molecules land and freeze on to the crystal, it grows.
0:09:20 > 0:09:24But they don't always hook on in the same way.
0:09:25 > 0:09:29Minute changes in temperature and humidity
0:09:29 > 0:09:31stamp their identity on the snowflake.
0:09:33 > 0:09:35By the time it hits the ground,
0:09:35 > 0:09:38each snowflake has been through a unique growth history.
0:09:45 > 0:09:48But it's how snow melts and re-freezes when on the ground
0:09:48 > 0:09:50that leads to avalanches.
0:09:58 > 0:10:01To examine how snow transforms, you need to make your own.
0:10:01 > 0:10:03Plenty of it.
0:10:05 > 0:10:06At the SLF,
0:10:06 > 0:10:10the Institute for Snow and Avalanche Research in Davos, Switzerland,
0:10:10 > 0:10:15they study snow crystals and what happens at their melting point.
0:10:19 > 0:10:23From a geological point of view,
0:10:23 > 0:10:26snow is a high-temperature material
0:10:26 > 0:10:29and that sounds very strange for most people
0:10:29 > 0:10:32because snow is almost a symbol for cold.
0:10:32 > 0:10:36But because snow is always very close to the melting point,
0:10:36 > 0:10:38it behaves as a high-temperature material.
0:10:40 > 0:10:47It's like a metal at several hundred or even a thousand degrees.
0:10:47 > 0:10:52And that makes it one of the fastest changing
0:10:52 > 0:10:55natural materials we see at all.
0:10:56 > 0:11:01Fresh snow can melt and re-freeze within the snow pack
0:11:01 > 0:11:05and it's this change of structure that can lead to an avalanche.
0:11:07 > 0:11:10To study how the snow changes in more detail,
0:11:10 > 0:11:13Martin designed a special CT scanner,
0:11:13 > 0:11:16a machine more commonly used in medicine
0:11:16 > 0:11:18to examine bones and tissues.
0:11:19 > 0:11:21Snow is a very elusive material.
0:11:23 > 0:11:31That made it very hard to really get a complete picture of the snow.
0:11:31 > 0:11:35And that was the state until about ten years ago,
0:11:35 > 0:11:38when we started this tomography.
0:11:38 > 0:11:42When we could really visualise snow in 3D,
0:11:42 > 0:11:48then we started to see snow in a very different way than before.
0:11:48 > 0:11:53The machine has enabled him to build up a 3D sequence of images,
0:11:53 > 0:11:56revealing how the snow structure evolves
0:11:56 > 0:11:58in the previously hidden detail.
0:11:59 > 0:12:02First I thought that must be great for everybody
0:12:02 > 0:12:06because now people can understand snow.
0:12:06 > 0:12:10But it doesn't look like the nice hexagonal,
0:12:10 > 0:12:13perfectly symmetric snowflakes.
0:12:13 > 0:12:15It looks simply strange.
0:12:17 > 0:12:21Martin then used the CT scanner to analyse snow
0:12:21 > 0:12:24taken from the mountainside immediately after an avalanche.
0:12:26 > 0:12:29So this sample is from an avalanche site.
0:12:29 > 0:12:32This block is only four millimetres wide.
0:12:32 > 0:12:36We see in this block the essential features.
0:12:37 > 0:12:41This big blob is re-frozen snow.
0:12:41 > 0:12:45So it got warm, but only a little bit.
0:12:45 > 0:12:51It created this huge crystal and that's the interface, you could say,
0:12:51 > 0:12:56between the hot upper layer and the weak layer.
0:12:57 > 0:13:01And the avalanche forms now somewhere in this weak layer.
0:13:06 > 0:13:10It's this weak layer that's at the root of most avalanches.
0:13:13 > 0:13:14The bonds between
0:13:14 > 0:13:18the large cup-shaped snow crystals in this layer
0:13:18 > 0:13:21are only weak and they break easily.
0:13:21 > 0:13:25And if such a weak layer sits in the middle of the snow pack,
0:13:25 > 0:13:29it becomes an avalanche waiting to happen.
0:13:39 > 0:13:40Once you've got a weak layer,
0:13:40 > 0:13:43all you need to start an avalanche is a trigger
0:13:43 > 0:13:47and over 90% of the deaths that are caused in avalanches
0:13:47 > 0:13:49happen in events that were triggered
0:13:49 > 0:13:51by the skier or the snowboarder themselves.
0:13:51 > 0:13:53And you can see it happening in this footage,
0:13:53 > 0:13:55which is amazing and dreadful in equal measure.
0:13:55 > 0:13:57This is a skier in Alaska.
0:13:57 > 0:14:00And you can see that he has triggered an avalanche.
0:14:02 > 0:14:06And if we look at it here, we can see that the snow pack
0:14:06 > 0:14:08has failed along this line.
0:14:09 > 0:14:12And you can see it even more clearly in this clip here.
0:14:14 > 0:14:17Scientists at the SLF devised an experiment
0:14:17 > 0:14:20that shows precisely what happens
0:14:20 > 0:14:23when just a small section of the weak layer is disturbed.
0:14:25 > 0:14:28And the place to watch is this bit.
0:14:28 > 0:14:30This is a big block of snow
0:14:30 > 0:14:31and there's a chainsaw from the other side
0:14:31 > 0:14:34that's cutting into the weak layer here.
0:14:35 > 0:14:37And the saw gets further and further along.
0:14:38 > 0:14:41Suddenly, the whole top block starts to slide.
0:14:41 > 0:14:45So what happens is, the scientist has cut away at the weak layer
0:14:45 > 0:14:48and that has failed and that has made the next bit fail
0:14:48 > 0:14:52and so all the way along the weak layer here has suddenly broken
0:14:52 > 0:14:56and all of that top layer is sliding off.
0:14:56 > 0:15:00So to trigger an avalanche, you just need this weak layer to fail.
0:15:00 > 0:15:02And even though an avalanche can have
0:15:02 > 0:15:05hundreds of thousands of tonnes of snow,
0:15:05 > 0:15:09just the weight of one skier can be enough to start it off.
0:15:10 > 0:15:12But in rare cases,
0:15:12 > 0:15:15something entirely different can trigger an avalanche.
0:15:15 > 0:15:18And this is hardly ever caught on camera.
0:15:25 > 0:15:27Oh, look at that! Look at that! Look at that!
0:15:27 > 0:15:29Straight ahead.
0:15:30 > 0:15:33Do you want to go in the tents or what?
0:15:33 > 0:15:35In April 2015,
0:15:35 > 0:15:38a huge earthquake struck in Nepal...
0:15:40 > 0:15:41Whoa! Whoa!
0:15:41 > 0:15:43..triggering a fatal avalanche
0:15:43 > 0:15:46that engulfed the base camp on Mount Everest.
0:15:57 > 0:16:01Tragically, 19 people lost their lives,
0:16:01 > 0:16:04making it the deadliest disaster on the mountain.
0:16:04 > 0:16:07- MAN:- Stay together. Stay together.
0:16:09 > 0:16:12But despite the devastation,
0:16:12 > 0:16:16the ground only got covered in a few centimetres of snow.
0:16:16 > 0:16:20In this avalanche, it didn't seem like it was the snow that killed.
0:16:25 > 0:16:28Avalanche expert Perry Bartelt
0:16:28 > 0:16:31simulates the forces behind avalanches.
0:16:32 > 0:16:34Avalanches are basically symbols of chaos
0:16:34 > 0:16:37and it's this chaos that makes avalanches so dangerous
0:16:37 > 0:16:41and why modelling avalanches is so particularly difficult.
0:16:42 > 0:16:45Perry wanted to find out exactly what happened on Mount Everest.
0:16:47 > 0:16:48When we heard the news
0:16:48 > 0:16:50that there were deaths at the Everest base camp,
0:16:50 > 0:16:52we thought the avalanche must have been immense.
0:16:54 > 0:16:57Using satellite pictures and photographs
0:16:57 > 0:16:59taken immediately after the event,
0:16:59 > 0:17:02Perry set out to calculate the size of the Everest avalanche.
0:17:04 > 0:17:07What we have here are photographs before the event
0:17:07 > 0:17:08and after the event
0:17:08 > 0:17:11and we've studied the photographs and what we immediately saw
0:17:11 > 0:17:14was that there was a large region here of ice
0:17:14 > 0:17:17located at about 6,000 metres high
0:17:17 > 0:17:19that was missing in the after photograph.
0:17:21 > 0:17:23- MAN:- The ground is shaking!
0:17:23 > 0:17:26It was this missing chunk of ice
0:17:26 > 0:17:28that caused the destruction on Everest.
0:17:33 > 0:17:38But at 50,000 cubic metres, it was surprisingly small for an avalanche.
0:17:40 > 0:17:44We were extremely shocked to see that such a small avalanche
0:17:44 > 0:17:46could cause so much damage.
0:17:47 > 0:17:50But because Everest is extremely steep,
0:17:50 > 0:17:54the avalanche accelerated to about 200km/h
0:17:54 > 0:17:57in just ten seconds.
0:17:57 > 0:18:00And it's what happened next that made it so deadly.
0:18:02 > 0:18:06Because it was running on ground which is very, very rough,
0:18:06 > 0:18:10it allowed the avalanche to take in huge amounts of air,
0:18:10 > 0:18:12probably millions of cubic metres of air,
0:18:12 > 0:18:15and grew to an incredible size.
0:18:15 > 0:18:18- MAN:- Do you want to go in the tents or what?
0:18:18 > 0:18:22This created an enormous powder cloud.
0:18:22 > 0:18:26Nearly 200 metres high, it inundated base camp.
0:18:26 > 0:18:28- MAN:- Do you want to go inside?
0:18:28 > 0:18:31And because it was so fast-moving,
0:18:31 > 0:18:34it brought with it a strong pressure wave
0:18:34 > 0:18:37that would have blown the mountaineers against the rock face.
0:18:39 > 0:18:43What you see here is the calculated impact pressure of the powder cloud.
0:18:43 > 0:18:46The red zone here signifies pressures
0:18:46 > 0:18:50that are dangerous to human beings standing outside.
0:18:50 > 0:18:52Down here you have the Everest base camp
0:18:52 > 0:18:55and you see that it is clearly in the red zone.
0:18:55 > 0:18:56If the cloud hits them,
0:18:56 > 0:18:59there is a good chance that they are going to be killed.
0:19:05 > 0:19:08Such a lethal powder cloud is very unusual.
0:19:09 > 0:19:11In most avalanches,
0:19:11 > 0:19:14it's not the powder cloud that causes all the devastation.
0:19:16 > 0:19:21The really deadly and destructive part of an avalanche is its core.
0:19:22 > 0:19:26It's the core where the mass of snow and ice sits
0:19:26 > 0:19:28and that annihilates everything in its path.
0:19:33 > 0:19:36But it's often hidden under a powder cloud,
0:19:36 > 0:19:38making it difficult to investigate.
0:19:48 > 0:19:50Which is why the pioneering scientists
0:19:50 > 0:19:53from the SLF have built the world's largest
0:19:53 > 0:19:57avalanche laboratory in a steep Swiss valley.
0:19:59 > 0:20:03Here, they artificially trigger avalanches with dynamite.
0:20:04 > 0:20:06EXPLOSION
0:20:09 > 0:20:11MAN SHOUTS IN FRENCH
0:20:24 > 0:20:28But when they first started their experiments in the 1990s,
0:20:28 > 0:20:31they set off an avalanche much bigger and more powerful
0:20:31 > 0:20:33than they'd bargained for.
0:20:38 > 0:20:40We were really surprised by the force
0:20:40 > 0:20:42by which the avalanche hit the shelter.
0:20:42 > 0:20:44We did not expect such a big thing to come down.
0:20:46 > 0:20:48This is a very massive and solid bunker.
0:20:48 > 0:20:50I think the walls are about 40 centimetres thick.
0:20:50 > 0:20:53Nevertheless, you could feel
0:20:53 > 0:20:56the vibrations of the whole building.
0:20:56 > 0:21:00LOUD CRASHING AND BANGING
0:21:00 > 0:21:02We heard a strong noise.
0:21:02 > 0:21:05That was because the door broke open and the snow came in,
0:21:05 > 0:21:08then the pressure in the shelter rose enormously.
0:21:08 > 0:21:10It was like diving into two metres of water.
0:21:14 > 0:21:16After the avalanche had hit us,
0:21:16 > 0:21:19we first had to try to get out
0:21:19 > 0:21:22because the shelter was completely covered by snow
0:21:22 > 0:21:26and so we started digging a tunnel out and then we had to work hard.
0:21:26 > 0:21:28It was really very compact snow.
0:21:32 > 0:21:33From this experiment,
0:21:33 > 0:21:36Dieter and his team were able to gain a better understanding
0:21:36 > 0:21:41of how the avalanche core behaves as it races down the mountain.
0:21:42 > 0:21:46Today, they're able to use far more sensitive equipment
0:21:46 > 0:21:49to get the most accurate picture of the avalanche core.
0:21:51 > 0:21:54And some of the best results come from radar.
0:21:55 > 0:21:57What we've got here is radar data
0:21:57 > 0:21:59from one of these large-scale experiments
0:21:59 > 0:22:02and what it shows is time going on along the bottom
0:22:02 > 0:22:05and then distance from the bottom of the mountain up the side
0:22:05 > 0:22:07and what you can see is here's the start of the avalanche
0:22:07 > 0:22:10coming down the slope as time comes on.
0:22:10 > 0:22:12But then it's followed by another strong line here
0:22:12 > 0:22:14and then another one here.
0:22:14 > 0:22:18What this is telling us is that the avalanche is coming down in stages.
0:22:18 > 0:22:19It's surging.
0:22:19 > 0:22:21And the only reason that we know
0:22:21 > 0:22:24that this is happening inside the avalanche core
0:22:24 > 0:22:26is that we've got radar data like this
0:22:26 > 0:22:28that lets us see past the powder cloud.
0:22:32 > 0:22:35This new data on avalanche surges
0:22:35 > 0:22:37also gives us a better understanding
0:22:37 > 0:22:40of what exactly happened to Kristoffer
0:22:40 > 0:22:41as he tumbled down the mountain.
0:22:46 > 0:22:47You can see from Kristoffer's footage
0:22:47 > 0:22:49that there was a point where he almost stops.
0:22:49 > 0:22:51It looks like he's safe
0:22:51 > 0:22:53and then another surge comes
0:22:53 > 0:22:55and carries him on further down the mountain.
0:22:59 > 0:23:02These surges are common in avalanches and they happen because
0:23:02 > 0:23:05the snow grains travel at different speeds at different heights,
0:23:05 > 0:23:08so the ones at the top tend to go faster
0:23:08 > 0:23:11and that means the top layer can overtake the bottom one.
0:23:11 > 0:23:14And then, while all of this is going on,
0:23:14 > 0:23:17the avalanche is incorporating more snow,
0:23:17 > 0:23:19so it's getting bigger
0:23:19 > 0:23:22and the thicker the avalanche core and the higher up it is,
0:23:22 > 0:23:24the more of these surges you tend to get.
0:23:53 > 0:23:58These new insights from models, experiments and lab analyses
0:23:58 > 0:24:02have given us a profound understanding of how snow changes...
0:24:03 > 0:24:06..what triggers an avalanche...
0:24:07 > 0:24:10..and how it develops as it hurtles downhill.
0:24:14 > 0:24:17And they're being used to improve the way we forecast risks
0:24:17 > 0:24:19and protect people.
0:24:29 > 0:24:34But despite these significant advances in avalanche science,
0:24:34 > 0:24:38there's still one thing that we can't predict.
0:24:38 > 0:24:41Exactly when and where one will strike.
0:24:44 > 0:24:46What can be done, however, is to stop
0:24:46 > 0:24:50uncontrollably large and life-threatening avalanches
0:24:50 > 0:24:52by triggering them while they're small enough
0:24:52 > 0:24:54to be safely neutralised.
0:24:59 > 0:25:01In the Alps alone,
0:25:01 > 0:25:06around 50,000 controlled avalanches need to be set off every year.
0:25:08 > 0:25:10After several days of snowfall,
0:25:10 > 0:25:15the risk of avalanches here in Grindelwald becomes extreme.
0:25:15 > 0:25:18It's ski patrolman Martin Matthis' job
0:25:18 > 0:25:23to trigger an avalanche before any more snow falls.
0:25:28 > 0:25:31TRANSLATION FROM HIS OWN LANGUAGE:
0:25:40 > 0:25:43He's taking 50 kilos of dynamite,
0:25:43 > 0:25:46enough to blow up several city blocks.
0:26:29 > 0:26:33Martin reaches the summit and sets the charge.
0:26:48 > 0:26:51EXPLOSION
0:26:54 > 0:26:58The explosion creates an air pressure wave in the snow pack,
0:26:58 > 0:27:00triggering a mini avalanche.
0:27:01 > 0:27:04But...it's not enough. He needs to go again.
0:27:19 > 0:27:21EXPLOSION
0:27:31 > 0:27:33Martin succeeds.
0:27:33 > 0:27:37This time, it's enough to cause the weak layer to fail
0:27:37 > 0:27:41and for the snow above to tumble downhill.
0:27:41 > 0:27:45This is the avalanche he needs to make the mountain safe.
0:28:12 > 0:28:15We associate avalanches with chaos and destruction
0:28:15 > 0:28:20as though an entire landscape is temporally out of control.
0:28:20 > 0:28:23But we can see now that they do have internal structure
0:28:23 > 0:28:25and predictable patterns.
0:28:25 > 0:28:28This is nature at its grandest but it's not random.
0:28:29 > 0:28:32We're not going to stop avalanches happening,
0:28:32 > 0:28:34but all this new science
0:28:34 > 0:28:39will let us understand and co-exist better with these gigantic events.