BBC_Krakatoa Revealed
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00:00On Boxing Day 2004, an earthquake displaced millions of tons of seawater, creating a massive
00:11tsunami.
00:17The wave swept out across Southeast Asia, destroying homes and killing over 300,000
00:22people.
00:24But it wasn't the first event of this kind.
00:30A century ago, a succession of much larger tsunamis wiped out the Indonesian coastline.
00:39These waves were three times higher than those seen in 2004.
00:47But they weren't caused by an earthquake.
00:53They were caused by an erupting volcanic island, an island called Krakatoa.
01:01Krakatoa has become a byword for disaster. It completely destroyed an entire mountain.
01:11Midday on the Sunday, there was an island here, and less than 23 hours later, the whole
01:15thing had just vanished beneath the sea.
01:18The bang was the loudest noise ever generated on the surface of this planet.
01:29Krakatoa is one of the best documented eruptions in history, but there have always been unanswered
01:34questions, such as how did the volcano generate such giant waves?
01:40Today, finding the answers is hugely important, because the volcano is back, and it's growing
01:47at a frightening rate.
01:51It will erupt again in the future, up on that scale, and next time it erupts, the effects
01:56will be much more severe.
02:00An eruption of this type today would lead to a loss of millions of lives.
02:10KRAKATOA
02:21The eruption of Krakatoa blew away over two thirds of the volcano. Only an outcrop of
02:26rock was left standing. The volcano appeared to have burnt out. But then, in 1927, that
02:33illusion was shattered.
02:40VOLCANO
02:43Violent eruptions broke the calm waters of the Sunda Straits. A cameraman rushed to the
02:48scene to capture a new volcano forming.
02:55There was probably volcanic activity beneath sea level almost immediately after the eruption
03:00in 1883. And then in 1927, eruptions broke through the surface, and they led to the formation
03:08of Anak Krakatoa, the child of Krakatoa, that is still growing. And it's now a healthy
03:14young teenager, a violent one, too.
03:24The new volcano is growing by almost 12 feet a year, and its rate of growth is accelerating.
03:29It is now over a thousand feet high.
03:33Geologists have no doubt it will erupt again. And when it does, the consequences could be
03:39even worse than last time.
03:42At the time of the 1883 eruption, the area was not particularly densely populated. Today,
03:49the area is very densely populated.
04:03Ten times more people live here than when Krakatoa last erupted. And more are on their
04:09way.
04:15The coast of Java is now being developed as a resort area. And if Krakatoa erupted again
04:22today with a similar type of eruption as it did in 1883, the effects would be devastating.
04:33If geologists are to help prevent this scale of devastation, they need to understand what
04:39happened in 1883.
04:42Okay, this is the hose, the compressor hose. What we're going to do, we're going to pump
04:46compressed air in from the compressor.
04:48Nick Petford, an expert in volcano collapse, has come to Indonesia to test a new theory.
04:55I first heard about Krakatoa when I was a child at school, six or seven years old, and
04:59I was absolutely fascinated by the idea, not just of the eruption, but the fact that
05:03a whole island could just more or less disappear from the face of the earth.
05:07With some help from the locals, Nick is about to re-enact the collapse of Krakatoa.
05:13Tell them to look at how tall I am, that's how big I want the cone.
05:18They're building a rough model of Krakatoa as it would have been in 1883. And they're
05:22using sand because Nick believes that volcanoes aren't, but are.
05:26And they're using sand because Nick believes that volcanoes aren't, as you might expect,
05:30made of solid rock.
05:33If you just hang on, I'm just concerned, you make it more shallow on this side.
05:38And if you look how a volcano forms, mostly they're made up from erupted products, lava
05:42flows, but also material that's flown up into the air and then falls back down around the
05:46cone. So on average, the properties of a volcano are not that different from a loose pile of
05:51sand, something with a small amount of cohesion or stickiness, but not, as most people think,
05:55built of solid, strong rock.
06:01It's looking pretty good. I think it's as high as it needs to be.
06:05All Nick now needs to do is recreate the pressure generated by millions of tonnes of molten
06:09rock, or magma, rising up from the core of the earth.
06:16What we're going to do is pump in compressed air via this tube, here's the compressor here,
06:20into the base of the volcano, or the pile of sand, and try and reproduce a deep-seated
06:24failure, which is the sort of thing we think may have happened in the final stages of the
06:29eruption at Krakatoa.
06:42With the compressor turned on, the pressure builds.
06:46Nick then signals for its release.
07:01Nick believes pressure from rising magma blew the volcano apart because it was already unstable.
07:16What's remarkable about this demonstration is how much it mirrors other explosive volcanoes.
07:29It's pretty impressive. What's happened is that the internal pressure at the base of
07:33the cone has resulted in a deep-seated failure, or landslide, inside the pile of sand.
07:38And we think exactly the same thing may have happened during the waning stages of the eruption
07:42of Krakatoa.
07:48And amazingly, what's left looks just like Rakata, the remnant of Krakatoa.
07:57To test his ideas, though, Nick needs to go to Anak, Krakatoa.
08:02But the newly formed volcano is a dangerous place.
08:06So first, he visits the local monitoring station.
08:10Can you ask him, has there been any recent volcanic activity?
08:17He said last week.
08:18Last week?
08:20Okay.
08:21So what I'm hoping to achieve in my study is a deeper understanding of the physical
08:25mechanisms behind volcano collapse and how we can predict if and when collapses will
08:30happen again.
08:32He said safe, but we must be careful.
08:34Safe, but be careful.
08:35Okay.
08:45The next day, Nick heads 30 miles offshore to the new location.
08:50He's going to meet a group of people who are going to be monitoring the eruption of
08:54Krakatoa.
08:56The next day, Nick heads 30 miles offshore to the new volcanic island.
09:03He intends to examine its structure and collect rock samples from its slopes.
09:11We can try and recreate what we think are the right conditions inside a computer.
09:15If we have sensible input information on the strength of the rocks, the kind of gas
09:19pressures you might expect, we can build three-dimensional models and we can work
09:24out exactly what conditions are required for failure in the volcano.
09:34Eruptions happen on a regular basis, and Nick is acutely aware of the risks.
09:40Whilst travelling in Indonesia, backpacker Lockhart Murdoch found out just how dangerous
09:44this island could be.
09:49Early one morning, Lockhart set sail with five friends to experience the thrill of seeing
09:53a live volcano.
09:59The next day, Nick and his friends set out to explore Krakatoa.
10:03They experienced the thrill of seeing a live volcano.
10:10The norm for people going was to set sail from Java about two, three in the morning
10:16and that way you'd get to the volcano before sunrise.
10:22We got to the volcano about five in the morning.
10:26The boat captain cut the engines off.
10:28We could just see the sort of silhouette of the volcano in the silvery moonlight and the
10:33waves reflecting the stars.
10:42And then we heard a great rumbling sound and the volcano erupted.
10:54It was quite thrilling, quite exciting.
10:59The eruption only lasts about 30, 45 seconds and after that I suppose we all thought, well
11:11we're going to settle down, wait another couple of hours and it'll go off again.
11:16But it was at that point this motorised rubber dinghy appeared.
11:20There was an Indonesian guy saying, come on, come on, and we looked at him and said, are
11:24you sure?
11:26He did, he said, yes, yes, come on.
11:29So we foolishly got on the dinghy and he took us ashore.
11:34We were thinking, you know, there's two, three hour gaps between eruptions so it was going
11:39to be OK.
11:42Once ashore, the boatmen suggested they climb to a ridge for a closer look.
11:47Once we'd taken our photos, looked around, we were then thinking about heading back down
11:52to the boat.
11:54And just at that point we felt this great rumbling in the ground.
12:03All of a sudden, with a crash and roar, there was a huge plume of black smoke and ash came
12:09out the top of the volcano.
12:17Lumps of ash and cinder were coming down on top of us.
12:23We had to run for it, basically.
12:29And as I was running I felt a rock hit me on the back of the head, other rocks hitting
12:33me on my back.
12:36And I fell over on my face.
12:41And looking to the side I saw there was a sort of medium-sized boulder there, so I took
12:45cover behind that.
12:47That protected the top half of my body but the bottom half was still getting bashed and
12:52pummeled by all the rocks.
12:55And one large rock, I think about the size of a basketball, landed on the back of my
12:59left leg.
13:03I was thinking, basically, what a horrible way to die, what a horrible way to die.
13:08A geologist working nearby took photos of the ash cloud that quickly enveloped the island.
13:22After about 30 or 45 seconds the eruption stopped.
13:27And there I saw my English mate, blood pouring from a head wound and obviously bashed up
13:32as well.
13:38The third member of our party lying on her back, badly injured.
13:44And the two of us basically had to carry her down the side of the mountain back to the
13:48shore to get on the boat.
13:55We'd been on the boat for about an hour or so.
14:00When the girl with the worst injuries died, I just felt numb at the time.
14:08Although Lockhart and the rest of his friends survived, they all suffered severe injuries.
14:15I mean, we were all amateurs.
14:17We didn't go there with any safety helmets or respirators or breathing equipment.
14:22We just went there as tourists, not realising the dangers, not realising the sort of raw
14:31power of the volcano.
14:33It's nature at its rawest, really, isn't it?
14:49Anak Krakatoa is regularly active.
14:52But even when quiet, the molten rock held deep underground makes the surface of the
14:58But even when quiet, the molten rock held deep underground makes the surface blisteringly
15:04hot.
15:09This heat, combined with a blazing tropical sun, makes Nick's climb to its summit particularly
15:16tough.
15:24It's actually quite strange being here.
15:26It's even rather a bit intimidating, in fact.
15:28It's very hot, very hard work.
15:31And I wouldn't say eerie, but there's almost a brooding presence here, a feeling about
15:37the place which makes you feel a bit uncomfortable.
15:45I think the original shape of Krakatoa in 1893 wouldn't have been too dissimilar to
15:49Anak Krakatoa is now.
15:51The point is the scale.
15:53The height of Anak now is 300 metres more or less above sea level.
15:56In 1883, the Krakatoa volcano would have been three times its height, twice the size of
16:00the Empire State Building.
16:10You can really smell the sulphur.
16:12This is amazing.
16:13Here we are on the rim of Anak Krakatoa.
16:16You can see evidence of previous eruptions, boulders, big ones and small ones, littering
16:20the floor.
16:21You can see the steam and sulphur and fumes behind me.
16:24Volcanoes monitor on four levels.
16:26Four is full eruption, more or less.
16:28This is level one, so it's relatively safe at the moment.
16:35Although it appears calm, there are occasional tremors and continual rock slides.
16:40But such activity is nothing unusual in Indonesia.
16:45This vast volcanic archipelago has over 100 active volcanoes.
16:52It's a dangerous place to live.
16:54But there are benefits.
16:56Because of the rich volcanic soil, crops grow readily.
17:03So in the 19th century, it was also a great place to make money.
17:07Coffee, tea and spices made Dutch colonists very wealthy.
17:14Most of the Dutch were plantation owners or government officials.
17:18One of the few scientists was Dr Roger Verbeek, a mining engineer,
17:23who wrote a definitive report on the disaster.
17:28His report essentially forms the basis of everything we know.
17:33And it made Verbeek a very famous man.
17:36And in his old age, he became a very famous man.
17:40And in his old age, he became a very famous man.
17:42And in his old age, he became a very famous man.
17:44He was referred to as essentially the father of the Krakatoa disaster.
17:53When the eruption began at midday on Sunday, August 26th,
17:57Verbeek was over 100 miles from the volcano.
18:00How far to Whitson's off?
18:01Three miles, sir.
18:02Quick as you can.
18:03Hey, hey, hey!
18:09Roger Verbeek was the right man in the right place at the right time.
18:14Sherman!
18:15He was able to forensically reconstruct the eruption
18:18from his own observations, from the accounts of sailors
18:21and from other people who had survived the experience.
18:24And he was able to give volcanologists today some idea of what happened.
18:29How could I have been so stupid?
18:32The answer's right in front of us. Look.
18:36Why didn't I see this at the time?
18:38Look at the profiles. Steep here and shallow down into the centre.
18:42You see? What does that tell you?
18:44A single common source of rock inside the...
18:46Inside the caldera. Exactly. The caldera. Singular.
18:49These aren't three different volcanoes. It's one.
19:02Verbeek interviewed a number of survivors.
19:04Amongst them, Johanna Bejerink, wife of a Dutch official.
19:11Even before the eruption began,
19:12Johanna seems to have appreciated the terrible things were about to happen.
19:21The animals know. They're leaving.
19:31They're leaving.
19:39When the volcano first erupted that Sunday, Johanna hurried home.
19:43There she received some unexpected news.
19:51One of the servants said to me in a very worried manner,
19:54the sea has gone, all the coral reefs along the coast are dried out.
19:58I said, how can it have gone? Perhaps it is low tide.
20:08The retreating sea was a warning that the volcano had triggered a tsunami.
20:16This is quite normal before a tsunami,
20:18because the first thing you see is the sea withdrawing
20:20and all the fish dancing about on the beach.
20:22And quite often people go down and say, great, we've picked those all up.
20:25They go down there and they all get clobbered by the wave coming in.
20:30The retreat of water is what happens before normal waves break on shore.
20:34But with a tsunami, the effect is massively amplified.
20:51I said get in the cupboard now.
20:52In the cupboard now.
20:53In the cupboard now.
20:55In the cupboard now.
21:14It's useless.
21:15Cupboards won't work in this.
21:18Johan Lindemann, captain of the Laudern,
21:20was sailing to the north of the volcano when it erupted.
21:23Soon, he was sailing blind through a dense ash cloud
21:26that had darkened the skies across the Sunda Straits.
21:33The lightning struck the main mast conductor five or six times.
21:37And the mud and rain which covered the masts, rigging and decks was phosphorescent.
21:43And then the rigging presented the appearance of St. Elmo's fire.
21:47They think it's evil spirits, sir, trying to sink the ship.
21:51This ash keeps falling, they'll succeed.
21:55What Lindemann and his crew were experiencing wasn't lightning in the normal sense.
22:01Whenever you have a big explosive eruption and you have this huge column of gas and ash,
22:05the ash particles become negatively charged and the gas becomes positively charged
22:09and that generates these electrical discharges and you start to generate light.
22:15You actually get people killed by lightning, in fact, during volcanic eruptions.
22:22Throughout that evening and overnight, Krakatoa continued to erupt.
22:28By Monday morning, an eruption column had risen over 25 miles above the volcano.
22:37By the morning of the 27th, the air was just so filled with ash
22:42that it was as if the sun didn't come up at all.
22:46All the lighthouse keepers on the west coast of Java simply kept their lights on.
22:52It was simply a day that the sun didn't rise.
23:01At five in the morning, it was the first in a series of huge eruptions.
23:06Then at two minutes past ten, a single massive explosion.
23:10It hurts my ears.
23:12The sound travelled almost 3,000 miles.
23:19To the west, it was heard just off the coast of Africa.
23:29To the east, it reached Alice Springs in the middle of Australia's outback.
23:34The explosion was the loudest sound ever recorded.
23:36It's almost impossible to imagine how loud it would have been.
23:38I mean, you can understand hearing an explosion, say, 30 miles away.
23:42300 miles away, a bit more difficult to understand.
23:44But 3,000 miles away is virtually impossible to think about.
23:47It's a bit like the volcano blowing up in New York and you can hear it erupt.
23:52It's a bit like the volcano blowing up in New York and you can hear it erupt.
23:55It's just a bit like the volcano blowing up in New York.
23:58It's a bit like the volcano blowing up in New York.
24:00It's the most difficult to think about.
24:01It's a bit like the volcano blowing up in New York
24:03and you can hear it simultaneously in Los Angeles and in London.
24:08Nick Petford thinks what made the noise so loud was the way Krakatoa collapsed.
24:14Well, I think the only way that you could have created such a loud explosion
24:18is by exposing a large amount of magma
24:21to atmospheric pressure more or less instantaneously.
24:24The magma chamber deep beneath Krakatoa contained vast amounts of molten rock and dissolved
24:32gas.
24:33And it's the gas that made Krakatoa so explosive.
24:38Now, magma chambers are a bit like cans of fizzy drink.
24:43In this can, there's gas dissolved under pressure.
24:45You can't see it, but you know it's going to happen when you open the lid.
25:05And the more quickly you release the pressure, the bigger the bang.
25:13But I think the main reason why the explosion was so loud is that some kind of landslip
25:18right towards the end of the eruption allowed this to happen, the sort of thing which we
25:21showed on the beach.
25:23OK.
25:24The
25:45final explosion and collapse of Krakatoa was followed by a series of tsunamis of quite
25:49staggering proportions that radiated out from the volcano.
26:03On that Monday morning, Captain Lindemann seems to be one of the few who realized what
26:07was going to happen.
26:13Lindemann decided that the only way that he was going to survive was to point his bow
26:18head on into the waves, to drop an anchor to give him some stability, and to put his
26:23motors to half a head so that he was counterpoised to these appalling waves.
26:34He kept at half speed in order to ride the sea as high as the heavens, which suddenly
26:48struck us and made us dread being buried under it.
27:02He survived.
27:03It was damn good seamanship on his part, and the log that he kept is the finest record
27:08from a mariner's point of view of what had happened.
27:15Although Lindemann and his passengers survived the tsunami, thousands didn't.
27:22When the wave collided with the coast, it tore a 60-ton piece of coral from the seabed
27:26and hurled it at the shore.
27:29In its path stood the Fourth Point Lighthouse.
27:46After its destruction, all that was left was the brick foundation.
27:56The giant waves battered the coastline, destroying 165 villages and killing over 34,000 people.
28:11But what created the tsunamis?
28:14Captain Lindemann thought it was seaquakes, underwater earthquakes.
28:20And that's how most tsunamis, like the Boxing Day one in 2004, are generated.
28:33But Roger Verbeek's report firmly stated that of Krakatoa, this wasn't the case.
28:41The air was made to vibrate so strangely by the tremendous detonations that one thought
28:46of earthquakes.
28:48But no earthquakes took place during this eruption.
28:55It started.
28:59Verbeek thought the tsunamis were caused by parts of the island falling into the sea.
29:04And for over a century, that's been the accepted wisdom.
29:08But not everyone agrees.
29:12One of the great mysteries of the Krakatoa eruption was what was the process by which
29:17the great tsunamis were generated.
29:21Volcanologists Steve Carey and Harald Erzigertsen had a very different explanation for what
29:27happened, but they needed proof.
29:30So they went to Anak Krakatoa to find it.
29:35Surprisingly, in the 100 years since the eruption, no one had bothered to look at what ended
29:40up in the ocean.
29:42And so this is where our work came in, is we wanted to find out what was the nature
29:46of the material on the seafloor.
29:52Neither of them were great divers, so they needed a location where they could easily
29:56collect samples.
29:59There's a ring on the seafloor, perfectly around the volcano, where there's a shallower
30:05region.
30:09These areas were accessible by scuba diving, so we thought this would be a great area to
30:14target to try to sample the material from the eruption.
30:19This required great stamina, as they were often making two or three dives a day in difficult
30:24conditions.
30:29We've done over 57 dives in the area, both to the north of Krakatoa and to the west of
30:34Krakatoa, where we've sampled the underwater deposits using a submarine coring device.
30:41We drove in pipes into the seafloor at about 65 locations.
30:49This was extremely hard work.
30:51Taking a core involves pounding this into the seafloor, and it's very difficult to move
30:57through water.
30:58It offers a lot of resistance.
31:02On some dives, the currents where we were working were so strong, we had to actually
31:06tie lead weights onto our feet to keep us from being swept away by the bottom currents.
31:15After two months of backbreaking effort, they collected enough samples.
31:20These were then shipped back to the States for analysis.
31:28In the lab, their hard work finally paid off.
31:32They proved all the old theories were wrong.
31:37Because what they'd found on the ocean bed wasn't rock debris from a collapsing volcano.
31:44It was something very different.
31:46They'd found a thick layer of what geologists call pyroclastic flow.
32:02Pyroclastic flows are really like the nuclear arsenal of volcanoes.
32:06They are the most destructive and lethal effects that we know of from explosive volcanic eruptions.
32:11They are hot particles and gas which flow down the slope of a volcano at hurricane speeds
32:18and with temperatures of 500 degrees centigrade.
32:28Pyroclastic flows are created when an eruption column suddenly collapses.
32:34The superheated ash, pumice and gas falls back to earth as a lethal avalanche of burning debris.
32:48Bill McGuire is a leading expert in natural hazards.
32:52In 1996 on Montserrat, he experienced the ferocity of pyroclastic flows.
33:03It was an opportunity to see these phenomena at close hand which nobody had ever had before.
33:07To be able to fly in a helicopter in front of one of these pyroclastic flows coming down towards us.
33:17The heat, even at a few hundred meters on your legs was just extraordinary.
33:20You had to check if they were burning or not.
33:22And you did think, well I hope the engines don't fail at this point, I must admit.
33:34You have this huge roiling cloud pouring down the side of the volcano.
33:38It's actually hugging the ground but it's generating all this hot gas and ash.
33:42So it's very very hot, you're talking hundreds of degrees Celsius.
33:45Sometimes they carry blocks the size of houses as well just to add to their destructive power.
33:50And really there is nothing you can do to mitigate their impact, there is nothing you can do to survive them.
34:04Although people are incinerated if they are caught in a pyroclastic flow, it's not the burning that actually kills them.
34:09It's the very very hot gas that they inhale.
34:12So the first inhalation will virtually destroy your respiratory system.
34:16And the second one will just finish the job.
34:19So people really drown in their own, in the liquid and the fluid generated in their lungs.
34:27Even those caught on the edge of a flow will be severely burnt.
34:33But most die.
34:47Krakatoa seems to have produced enormous amounts of pyroclastic flow.
34:55Even Haraldur and Steve were astonished by the amounts.
35:00We were able to determine the amount of pyroclastic flow material deposited on the ocean is up to 80 metres thick to the west of the volcano
35:08and about 40 metres thick to the north of the volcano.
35:13They'd found enough material to cover central London in a layer 500 metres thick.
35:18Based on these findings and historical records, we can now reconstruct a likely sequence of events.
35:29A giant landslide on the morning of the 27th exposed vast quantities of magma.
35:37This caused a massive explosion and an enormous blast of sound.
35:50The eruption column then collapsed.
35:55This created pyroclastic flows that hurtled down the sides of the volcano and crashed into the sea.
36:05When these very large pyroclastic flows went into the ocean, they displaced the water.
36:10And that displacement of the water is what generated tsunamis.
36:16After more than a century, they had finally found the answer to what had caused the tsunamis.
36:25It wasn't collapsing rock, but pyroclastic flow.
36:40And this breakthrough helped explain another mystery that had until then remained unsolved.
36:47One of the real fascinating aspects for us about the Krakatoa eruption
36:53was observations that were made on the south coast of Sumatra here at Katimbam,
36:58where about 2,000 people were killed by burns.
37:02This is very unusual because Katimbam is over 40 kilometres over open ocean from Krakatoa volcano.
37:10The most graphic account comes from Johanna Beyerink.
37:21The last thing I saw was the ash being pushed up through the cracks in the floorboards like a fountain.
37:28Then it seemed as if the air was being sucked away and I could not breathe.
37:34Mrs. Beyerink described hot ash which actually came up through the floorboards,
37:39forcibly came up and burned the people who were in the hut.
37:42This was not material that was falling out of the atmosphere.
37:45The only way that this could occur, if it was some kind of laterally moving flow,
37:49travelling at high speeds in order for it to actually physically push hot ash up into the cabin.
37:56This could only be a pyroclastic flow.
37:59But how could hot ash and rock cross 30 miles of ocean and then two miles of land?
38:05We know a lot about pyroclastic flows and the fact that on land
38:09they can travel great distances away from the volcano.
38:12The problem is we really didn't understand how pyroclastic flows behave once they interact with water.
38:18They would go underwater, they would go over the surface of the water,
38:22they would explode by some kind of complex interaction.
38:25So the behavior was very poorly understood.
38:29Although Haralda and Steve had proved that Krakatoa had produced pyroclastic flows,
38:34no one had studied how it could surge across water.
38:40The Krakatoa was a very large volcano.
38:43No one had studied how it could surge across water.
38:52At Kiel University in Germany, geologist Armin Frundt agreed to do an experiment.
38:59The Krakatoa eruption was located in the sea,
39:02so the eyewitness accounts were mostly taken from a great distance
39:06and don't provide direct information about the eruption.
39:12So the experiments are a good way to visualize and then investigate these processes.
39:20Armin is about to recreate what happened in 1883.
39:28First, a hopper filled with superheated ash is raised above a chute.
39:36At the bottom is a tank of water.
39:38When the ash is released, it will surge down just as the pyroclastic flow did at Krakatoa.
39:47But what will happen when hot ash meets the water?
40:09It's all over in seconds, but using high-speed cameras means we can see in detail what happened.
40:18The hot ash divides, the heavy part sinks, creating a wave.
40:25This is the tsunami.
40:31But the lighter ash blasts across the water,
40:35It does this by riding on a bed of steam.
40:42Even more surprisingly, it seems to be self-propelling.
40:48An exciting result from our experiments was that when hot volcanic ash hits the water,
40:55the water is turned to steam and a steam eruption is produced.
41:00That speeds up the pyroclastic flow so that it can reach greater distance from the volcano.
41:07The steam produced by the hot ash not only keeps it above the water, but actually propels it forward.
41:20So, it's reasonable to assume that such an event is a natural phenomenon.
41:27So, it's reasonable to assume that such steam eruptions also occurred during the Krakatoa event.
41:43When pyroclastic flows surged down the volcano, part went underwater, creating tsunamis.
41:50The rest flowed across the ocean.
41:53Thrust forwards by steam eruptions, they crossed the Sunda Straits at speeds of up to 150 miles an hour and then hit the land.
42:14We know from eyewitnesses that within 15 minutes of leaving the volcano,
42:18it killed 2,000 people on the coast of Sumatra.
42:25Johanna Bejarink and her family survived because they found shelter.
42:32Those caught outside were not so fortunate.
42:36The eruption was over within a day, but its effects were far-reaching.
42:41Within hours, news was being telegraphed across the world,
42:45making Krakatoa the first natural disaster of the modern age.
42:53Tide charts recorded that the tsunami travelled as far as the island of Krakatoa,
42:58and that it was the largest tsunami in the world.
43:02Tide charts recorded that the tsunami travelled as far as Europe.
43:09Fine ash from the volcano thrown high into the atmosphere created extraordinary sunsets around the globe,
43:16and by blocking sunlight, caused world temperatures to drop well into the 20th century.
43:22Krakatoa, the volcano which became a byword for disaster, has now returned.
43:28The fear is that one day Anak Krakatoa will violently erupt like its parent, generating pyroclastic flows.
43:37And that's why Nick Petford is here.
43:40His measurements and rock samples will be used to predict which parts of the coast are in danger.
43:45There's very little sign of weathering.
43:50I'm just going to prise a piece off.
43:53The rock is pretty tough, but there's some loose bits here.
43:58So I'll take this bit here, which comes out.
44:03It's quite loose.
44:05And we'll take this back to the aboriginal people.
44:08Back in London, Nick is working out the unusual behaviour of the volcano.
44:13He's been called to a few different places.
44:16But in the end, they've all been fine.
44:19The volcano has been around for a year.
44:22It's been called a wonderland.
44:25It's been called a wonderland.
44:28And it's been called a wonderland.
44:31It's been called a wonderland.
44:33Back in London, Nick returns to the lab with his samples and measurements.
44:40This is crucial data for colleagues recreating Anak Krakatoa in the computer.
44:49By cutting and crushing the rocks, the strength of the volcano can be estimated.
45:03These allow them to recreate the shape of the volcano, and they also input data from
45:08other volcanoes, such as the pressure caused by rising magma.
45:13Using all this information, Nick's team have managed to make a prediction about the
45:17likely outcome of a future eruption.
45:23If it did fail, it's more than likely to fail towards the southwest, which is 180 degrees
45:29different from the failure direction which took place at the final cataclysmic eruption
45:34in 1883.
45:35And luckily, that takes most of these flows away from any large areas of population and
45:39away from the more densely populated coastline of western Java.
45:46This is unexpected good news, though a major eruption would still put people's lives
45:51at risk.
45:52But it may be possible to evacuate coastal areas if they have enough warning.
45:58Anak Krakatau will erupt at some time in the future, so it's really important that
46:02we carry on monitoring the volcanoes quite closely as the volcano evolves and grows.
46:09Understanding the events of 1883 allows geologists to prepare for future eruptions.
46:14Yet, in Indonesia, Krakatoa has largely been forgotten.
46:18I found it extraordinary on a recent visit out there to find that many of the people
46:23living in western Java and southern Sumatra had barely heard of Krakatoa.
46:27People have just forgotten already that there was this catastrophic event which must have
46:30killed many of their ancestors.
46:35And even those who know the Krakatoa story seem unconcerned.
46:40I like it, but, well, I'm not really afraid right now because it's not dangerous like
46:49100 years ago.
46:52I don't worry about the Krakatoa because I work for tourism.
46:57I get money because I made a night trip and sunset cruise and we stay overnight at the Krakatoa.
47:06It's very fantastic when the Krakatoa is active.
47:12One of the casualties of the eruption was a Dutch warship, a small ironclad vessel called
47:19the Berow, which was picked up by the wave and carried fully two and a half miles inland
47:25and dumped in a river valley.
47:27That ship remained there, gradually rotting away, and it's all completely gone now, completely
47:33disappeared except for one thing, the mooring buoy from which it was torn.
47:39It stands now in the middle of a traffic island in Telok Betong in Sumatra, passed by thousands
47:47of motorists and bicycles and pedestrians every day, almost unnoticed, and yet that's
47:52the only memorial to the 36,000 people that were killed.
48:02Krakatoa was a global event that shook the world, killed tens of thousands and made the
48:07name synonymous with disaster.
48:10But it's not just of historical interest.
48:13Today when half a billion people around the world live in the shadow of a volcano, understanding
48:18how they erupt is more important than ever.
48:22And that's what understanding Krakatoa allows us to do.
48:31Next tonight, a pilot, a cameraman, a rickety wicker basket and the world's tallest mountain.
48:35Settling old scores as Chris Dewhurst and Leo Dickinson take up a grudge match in the
48:39final part of Endeavour Everest.