The Toba volcano is a super volcano in the Barisan Mountains of Sumatra, Indonesia that last erupted around 74,000 years ago. The Toba volcano is one of the largest on Earth, with a caldera that's 35 x 100 km. The Toba eruption is considered one of the largest volcanic eruptions in human history, releasing an estimated 2,800 cubic kilometers of ash and lava. It was 100 times larger than the 1815 eruption of Mount Tambora, which caused the "Year Without a Summer".
The eruption had many effects, including:
Climate change: The eruption may have sent the planet into a severe ice age.
Ozone depletion: The eruption caused a collapse in tropical stratospheric ozone, which increased the maximum daily UV index by 140%.
Genetic bottleneck: Some scientists believe the eruption may have decreased the human population to a few tens of thousands of individuals. However, this hypothesis is not widely accepted.
Lake Toba
The volcano's caldera is now filled with water, forming Lake Toba, the world's largest volcanic lake.
The Toba eruption (also called the Toba supereruption and the Youngest Toba eruption) was a super volcanic eruption that occurred about 74,000 years ago, during the Late Pleistocene, at the site of present-day Lake Toba, in Sumatra, Indonesia. It was the last in a series of at least four caldera-forming eruptions there, the earlier known caldera having formed about 1.2 million years ago.[3] This, the last eruption had an estimated volcanic explosivity index of 8, making it the largest known explosive volcanic eruption in the Quaternary, and one of the largest known explosive eruptions in the Earth's history threatening the extinction of the cavemen.
#ancient #prehistoric #documentary
The eruption had many effects, including:
Climate change: The eruption may have sent the planet into a severe ice age.
Ozone depletion: The eruption caused a collapse in tropical stratospheric ozone, which increased the maximum daily UV index by 140%.
Genetic bottleneck: Some scientists believe the eruption may have decreased the human population to a few tens of thousands of individuals. However, this hypothesis is not widely accepted.
Lake Toba
The volcano's caldera is now filled with water, forming Lake Toba, the world's largest volcanic lake.
The Toba eruption (also called the Toba supereruption and the Youngest Toba eruption) was a super volcanic eruption that occurred about 74,000 years ago, during the Late Pleistocene, at the site of present-day Lake Toba, in Sumatra, Indonesia. It was the last in a series of at least four caldera-forming eruptions there, the earlier known caldera having formed about 1.2 million years ago.[3] This, the last eruption had an estimated volcanic explosivity index of 8, making it the largest known explosive volcanic eruption in the Quaternary, and one of the largest known explosive eruptions in the Earth's history threatening the extinction of the cavemen.
#ancient #prehistoric #documentary
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00:00Disaster has stalked the Earth since the birth of the planet.
00:09The result?
00:1099% of all species that have ever existed are now extinct.
00:20Our home planet has been shaped by vast forces throughout history.
00:27These forces are still as powerful today as ever.
00:32So far we humans have been lucky.
00:37We haven't experienced a global disaster.
00:42And yet history reveals we're much more vulnerable than we think.
01:10To understand the sheer scale of Earth's four and a half billion year history, imagine
01:15it as the 24 hours on a clock.
01:21Earth formed at midnight.
01:23Just nine minutes later disaster struck.
01:26Our planet collided with another.
01:33But Earth survived and life evolved.
01:39Then at 8.30 p.m. another disaster.
01:43The entire planet froze over.
01:4610.40 p.m. volcanic eruptions poisoned the planet.
01:56And at 11.38 p.m. a giant meteorite killed off the dinosaurs, leading to the rise of
02:02the mammals.
02:08It was only in the closing minutes of the day that our planet became a place we'd recognize.
02:18Finally at under a minute to midnight, a tough new species marched towards world domination.
02:35They spread rapidly, adapting to every challenge.
02:41This new species was Homo sapiens, us.
02:4685,000 years ago we were just heading out of Africa.
02:52Today we're everywhere.
02:56This is the story of what happened since humans walked the planet.
03:00And it shows just how vulnerable we really are.
03:04Again and again our ancestors confronted catastrophes.
03:08All of them were different.
03:10But any one of them could have stopped human civilization in its tracks.
03:19The first disaster struck India 74,000 years ago.
03:25Even today evidence for this event can be found in the most unlikely place, inside the
03:31cells of our bodies.
03:36The story of the human race is written in our genes.
03:41Our genes control not only what we look like, they also record evidence of past disasters.
03:50For geneticist Stephen Oppenheimer it's a crucial clue.
03:57As you move away from Africa the overall genetic diversity reduces in different populations
04:03until you get to the Native American populations which have the least diversity of all.
04:11But there is one place where there's an anomaly, that's India.
04:18In India genetic diversity is much, much lower than it should be.
04:25Oppenheimer believes some kind of disaster must have struck India's early settlers.
04:31Something so severe that their descendants' genetic diversity is still affected today.
04:38Whatever it was, this ancient disaster came close to wiping out the whole subcontinent.
04:47It's difficult to estimate the size of reduction, but it might have been down to about 600 people
04:53in the whole of India.
04:58Whatever struck India, it was absolutely devastating.
05:04Something powerful enough to wipe out most of the population.
05:09There is of course a very obvious catastrophe which is clearly dated in the right time zone
05:18and that is Toba.
05:23Toba is an Indonesian supervolcano.
05:28Its last eruption is described by volcanologists as mega-colossal.
05:33That's as big as it gets.
05:36The date, 74,000 years ago, the estimated time of the Indian disaster.
05:44Was it a coincidence?
05:46Or was it the catastrophe that nearly killed off India's people?
05:56Volcanoes are one of the most powerful forces on the planet.
05:59They can devastate whole regions and even affect global climate.
06:15This is Mount Augustine off the coast of Alaska.
06:18It's not a supervolcano, but it does illustrate the raw power of even a fairly small eruption.
06:27It last blew in 2006.
06:32Volcanologist John Power is monitoring how it's changed since then.
06:45During the last eruption here, Augustine blasted out around 2 billion cubic feet of rock.
06:53So much debris spewed out that the summit grew by over 200 feet.
07:07The eruption here was big, but it was nothing compared to the power that could be unleashed
07:12by a supervolcano.
07:17Studying smaller eruptions like Augustine gives scientists an insight into the incredible
07:22power of the Toba eruption 74,000 years ago.
07:32We're sitting on Augustine Island, which is the home of Augustine Volcano, which you see
07:35behind us.
07:37We are at the very northern end of what's called the Ring of Fire.
07:42The Ring of Fire is a chain of volcanoes that surrounds the Pacific Ocean.
07:48It's the world's most volcanically active region.
07:52Toba lies in Indonesia at its western edge.
07:59Here at many of these volcanoes in the Ring of Fire, you have very explosive types of
08:03eruptions, very powerful things that throw ash and so on out in very high elevation in
08:08the atmosphere.
08:10There's no place where things are quite as active as Indonesia.
08:15Indonesian volcanoes have produced some of the most violent explosions on the planet.
08:22The Toba eruption was the biggest on earth for two million years.
08:29The forces are quite extreme during one of these large explosive volcanic eruptions.
08:34You have magma that's coming up underneath the volcano.
08:39Inside that magma you have a lot of gas and so on that's absorbed inside the magma itself.
08:44And it's really this gas pressure that is driving a lot of the eruption.
08:50An average volcano might have enough gas and magma for the eruption to last for some hours.
08:58Toba would have erupted for days.
09:06But while such eruptions are rare, the volcanoes that cause them aren't.
09:1347 supervolcano sites have been discovered worldwide.
09:18Many are no longer active, but a few are, and they pose a real threat to human society.
09:26The most famous one of all lies in the United States.
09:49Yellowstone
09:53This bizarre landscape attracts over three million visitors every year.
10:02They come to enjoy the scenery and witness the raw power of the park's famous geysers.
10:24Yellowstone contains the largest collection of such hydrothermal features anywhere on earth.
10:32Two-thirds of the world's geysers are in this one park.
10:38That takes a lot of heat.
10:40In fact, it takes a supervolcano like Toba only hidden below ground.
10:52The last super-eruption here was 640,000 years ago, long before humans ruled the planet.
11:00But even after all this time, you can still see evidence of this ancient blast.
11:06And the volcano itself remains active.
11:09One day it will erupt again.
11:15For geophysicists like Bob Smith, Yellowstone is a vital research center.
11:23For decades, Smith has been studying the Yellowstone caldera,
11:27the giant volcanic crater in the center of the park.
11:32His work reveals just how devastating the eruption of Toba would have been.
11:40We're standing here on the east side of Yellowstone Lake,
11:43and the sharp hill in front of us is actually the caldera boundary.
11:48And the caldera essentially occupies this entire expanse of the landscape that we can see.
11:55This whole system exploded out during the last giant eruption.
11:59This is a giant caldera, probably one of the biggest in the world that's known, and this is active.
12:07The sheer size of the Yellowstone system makes it a key location
12:11in the study of the ancient Toba super-eruption.
12:17The Yellowstone caldera compares to Toba roughly in the same dimensions,
12:22of about 60 kilometers by 40 kilometers.
12:27Toba has a large lake occupying the caldera as Yellowstone Lake, so it's very similar in size.
12:36These hills and rocks were sculpted by immense forces.
12:42The whole landscape has been shaped by the giant volcanic furnace below.
12:50The magma chamber.
12:53This is the zone where molten rock gathers under immense pressure deep below the caldera.
13:01The bigger the magma chamber, the deadlier the eruption.
13:11Smith's work here at Yellowstone shows Toba's magma chamber would have been huge.
13:22This is his laboratory.
13:28We're here at a site on the east side of the caldera, where we have a seismograph,
13:34which records ground motions that relate to the vibrations of the earth
13:38when we have the passage of seismic waves.
13:41So we record 2,000 to 3,000 earthquakes a year here.
13:48By mapping his seismic data, Smith can estimate the size of the magma chamber.
13:55His results are stunning.
14:01This simulation shows the whole United States.
14:06Yellowstone Park lies near the middle, its boundary marked in green.
14:14Yellowstone Lake is marked in blue, the edge of the volcanic caldera in red.
14:22Smith's seismic data, plotted below the surface,
14:27shows the enormous size of the magma system beneath the caldera.
14:35Yellowstone's magma chamber is an astounding 15 miles wide by 31 miles long.
14:42It's five miles deep.
14:46It's 20 times the size of the island of Manhattan.
14:52If this much magma erupted again, the consequences would be immense,
15:00just as they were 74,000 years ago, when Toba blew.
15:07Just over a second to midnight on our clock of human history,
15:18the Indonesian supervolcano Toba had just erupted.
15:25Our ancient ancestors faced a terrible threat.
15:32But not from red-hot magma. The real killer was volcanic ash.
15:39When the magma actually finally makes it to the surface,
15:43the gas pressure will drive that magma, fracture it,
15:46pulverize it into what a volcanologist would call ash.
15:49This is pulverized rock and minerals, all ground up together by the explosive forces.
15:55That stuff can be thrown out into the atmosphere to great altitude.
16:01It's thought that Toba's eruption column reached the very edge of space.
16:07This footage from the space shuttle of the eruption of Russian volcano Mt. Plutchevskoi
16:12shows how high ash can be thrown into the atmosphere.
16:18But at Toba, 74,000 years ago, this was just the beginning.
16:26As all those gases and pulverized rock rise up,
16:29it's hot, very hot, about 1,100 degrees centigrade.
16:32When it comes out, it'll rise up first buoyantly under its own heat.
16:36And as it begins to cool, it will become too heavy for the atmosphere to support.
16:40And it will rush back down the sides of the volcano.
16:44This creates a very hazardous phenomenon that we refer to as a pyroclastic flow.
16:52These superheated ash flows can be immense.
17:00At Toba, they buried the landscape 600 feet deep.
17:09Any humans nearby would have been annihilated.
17:15But even those outside this initial danger zone weren't safe.
17:22Toba's volcanic ash traveled for thousands of miles.
17:27There was a massive release of ash.
17:32And that ash went northwest in the Indian Ocean and covered India.
17:40Twelve and a half million square miles of the Earth's surface were covered in ash.
17:48Anyone living in the fallout zone faced starvation.
17:55The Toba ash fall would have affected the vegetation in a big way in India.
18:01And the immediate effect of that would be that the game that humans relied on
18:07didn't have any vegetation to eat.
18:11And then of course the human predators, being at the top of the chain, suffer much more.
18:20The ash was deadly.
18:23But volcanoes have an even deadlier weapon in their arsenal.
18:30The gas, sulfur dioxide.
18:36Toba may have released as much as three billion tons of it.
18:45Volcanologist Bill McGuire has studied how sulfur dioxide can affect the entire planet.
18:53When sulfur dioxide gets into the atmosphere, which it does with a big volcanic eruption,
18:57it combines with water vapor and it forms a fine mist of sulfuric acid.
19:03The result? The planet cools down and enters a volcanic winter.
19:23There's some debate about how much of a temperature fall Toba actually led to.
19:27But in the extreme case it could have reduced global temperatures by five to six degrees
19:32centigrade for a period of several years.
19:36And that would have literally caused most of the world's vegetation to die off.
19:43The effects of another super eruption today hardly bear thinking about.
19:52Starvation would wipe out huge numbers of people.
19:58If we saw a super eruption today that resulted in that same temperature drop,
20:02then we would experience global harvest failure.
20:04I can't see any way that that can not result in billions of deaths.
20:12If another of Earth's active super volcanoes does what Toba did 74,000 years ago,
20:18it would be a disaster for us all.
20:26Super eruptions on average seem to occur about every 50,000 years or so.
20:31But of course the Earth doesn't operate to a timetable.
20:36So when the next one's going to occur, we really haven't a clue.
20:40Toba wiped out great numbers of people in India.
20:47But 21,000 years ago the survivors faced another threat.
20:52This time one that affected the whole planet.
20:58A global big freeze.
21:07Half a second to midnight on our clock of world history, 21,000 years ago,
21:14the planet was in the middle of an ice age.
21:22Throughout history, ice ages have shaped the story of our planet.
21:29The biggest of all came 650 million years ago, when Earth was nearly consumed by ice.
21:38Our planet had a lucky escape,
21:44when volcanoes broke through and warmed the planet again.
21:51But in the millions of years that followed, the ice sheets frequently returned,
21:56just as they will in the future.
22:0321,000 years ago, Earth was gripped by the most recent of these big freezes.
22:09Glaciers steadily advanced across the northern hemisphere.
22:14For our ancestors, there was no escape.
22:23Glaciations occur on a regular cycle,
22:25caused by variations in Earth's movement through space.
22:31Sometimes Earth moves further from the Sun,
22:34so the planet cools and the ice caps expand.
22:40Glaciers of the last ice age reached their furthest point south 21,000 years ago,
22:46a period known as the last glacial maximum.
22:50The last glacial maximum in Europe was about as bad as it can get.
22:55And that meant an ice cap three miles thick, which covered half of Britain.
23:01And around that ice cap to the south was a polar desert,
23:05which didn't have ice on it, but also didn't have much vegetation or people.
23:13For our ancestors, it was migrate or perish.
23:19They didn't return until the Big Thaw began, around 7,000 years later.
23:27And when the glaciers retreated, it changed everything.
23:36Released from the grip of the ice, civilization was finally free to begin.
23:44Agriculture, cities, the Industrial Revolution, banks, well, you get the picture.
23:52The end of the ice age made all of it possible.
23:56But the irony is, it's all so complex that we'd be helpless if the glaciers advanced again.
24:05And if there's one thing that's certain, it's that one day the ice will return.
24:13Even now, Earth's orbit is taking it further from the sun.
24:18Another glacial advance is due any time.
24:27The return of the ice would be a brutal shock.
24:32In 1998, we got a glimpse of just how brutal.
24:38An ice storm hit the city of Montreal.
24:42Freak weather conditions created a relentless build-up of ice.
24:471,000 electricity pylons collapsed under its weight.
24:53The power supply failed.
24:57Millions of inhabitants were left without heating.
25:02And the temperature continued to rise.
25:07The ice was melting faster than ever before.
25:13Millions of inhabitants were left without heating.
25:20And the temperature continued to fall.
25:27The Montreal ice storm exposed our society's Achilles' heel.
25:32Our reliance on near-perfect conditions.
25:37If the glaciers were to advance again, there's not much we could do to protect ourselves.
25:45Inevitably, a new ice age, if it occurred very rapidly, would lead to complete social and economic breakdown.
25:52People would move towards the equator from northern countries like the United States, the UK, Europe.
25:57That would be a recipe for war and conflict without any doubt.
26:02A civilization that makes our lives so comfortable also makes us vulnerable.
26:11But 13,000 years ago, that's less than a second to midnight on our clock, our Stone Age ancestors were luckier.
26:19They had migrated south and survived off the land.
26:24Humans had now made it through a super-eruption and an ice age.
26:29But the risk of another disaster seemed remote.
26:38Sadly, it wasn't.
26:41The planet had done its worst, but there was still space to be reckoned with.
26:47Asteroids have struck Earth throughout history.
26:50In the ancient past, they even caused mass extinctions.
26:59Scientists are now beginning to wonder if humans could also have been affected by a cosmic catastrophe.
27:09This is Ohio, USA.
27:14This is the site of a major catastrophe.
27:19Something happened which had a profound effect on the life of the times.
27:26Archaeologist Ken Tankersley believes that at the end of the last ice age, 13,000 years ago,
27:33this region suffered a catastrophe that originated in space.
27:41These days, most of Ohio is farmland, but one small area of marsh remains.
27:48It's just as it was 13,000 years ago.
27:53Except for these.
27:59Back then, the area was home to an impressive selection of beasts.
28:05There were mega-mammals roaming this area, which included mammoths and mastodons.
28:12These mega-mammals were food for the continent's top predators, humans.
28:21The people who lived here, we refer to as Clovis.
28:26They were Stone Age hunter-gatherers.
28:29They were hunting wild game and gathering wild plant foods,
28:33and living in the wild was a very difficult task.
28:37They were hunting wild game and gathering wild plant foods,
28:41and living in extended families.
28:48These people had successfully adapted to the landscape for thousands of years,
28:55and then came a catastrophe.
28:58The mega-mammals went extinct.
29:01Their livelihood was gone forever.
29:08Clues to the cause of this catastrophe lie 30 feet below ground.
29:16This is Sheridan Cave. It's a natural time capsule.
29:23Its secrets might solve the mystery of the missing mega-mammals.
29:29Tankersley's work in the cave has unearthed a treasure trove of archaeological remains,
29:35all of them dating to the time of the disaster.
29:42It's a long descent to the bottom of the cave,
29:46and a journey back in time.
29:52Recent excavation in the cave has revealed a large number of remains,
29:57and this cave has revealed a dull red layer,
30:00which marks the exact moment that the mega-mammals vanished from the fossil record.
30:06It's known as the Clovis layer.
30:12We're looking at the Clovis layer.
30:15It's a very distinct layer here in the cave.
30:18Beneath it, we have mega-mammal remains.
30:21Above the layer, there are no more mega-mammals.
30:24It literally represents the extinction event.
30:28And you can find the same thing at more than 20 other sites across America.
30:35The sediment layer marks the exact moment the mega-mammals disappeared.
30:43One of the things that intrigues me about this time period and about this site
30:48is we have no clear-cut answer as to what caused the extinction of these mega-mammals.
30:55Over-hunting, people killing these animals, just does not fit.
31:00And when we look at all the other ice ages which came to an end,
31:04these mega-mammals did not go extinct.
31:07So why now, and why here?
31:10This is one of the most intriguing questions that I've ever faced.
31:18Excavation here continues.
31:21Even after a decade, they're still digging up bones.
31:27Even though this bone looks fresh, it is actually 13,000 years old.
31:33It dates to the extinction event.
31:37And it suggests a violent death.
31:40What's really exciting about this particular specimen
31:44is that there's clear evidence of burning, literally of blackened color.
31:52This is the tibia bone of a now-extinct pig-like creature,
31:56the size of this modern-day wild boar.
32:01In order to burn the flesh off of an animal the size of a European wild boar,
32:07we're talking about temperatures between 300 and 600 degrees centigrade.
32:13This is not an animal that was subjected to a cooking fire.
32:19This animal was incinerated, and so was the entire landscape.
32:27We're talking about a massive fire, almost an explosion of heat and pressure.
32:35The question is why?
32:4313,000 years ago, disaster struck America.
32:48The mega-mammals were wiped out,
32:50and the people who hunted them lost their livelihoods.
32:59The cause of this disaster has long been a mystery.
33:02But deep in these Ohio caves, archaeologist Ken Tankersley
33:06has discovered something which might provide the answer.
33:13The Clovis layer is marked by this bright orange layer.
33:17The big question is what's causing the red.
33:20One possibility is iron, and there's a wonderful test for that.
33:25This is a meter which measures the magnetism,
33:29the amount of iron, the higher the iron content,
33:32the greater the magnetic susceptibility of this layer.
33:38I'll first put the probe in this gray area.
33:42Below the Clovis layer is a perfect spot.
33:46And we check the magnetism.
33:49We see that it has a magnetism of 8.
33:53Now what we're going to do is compare that with the layer above it.
34:01The reading is 50 times the iron content.
34:06In other words, the magnetic susceptibility is 50 times higher
34:12than the area that's gray.
34:19A basic experiment reveals just how rich in iron the Clovis layer really is.
34:26A magnet dragged across the surface is left covered in iron particles.
34:32It's a simple test with an astonishing implication.
34:36It suggests this region was hit by an asteroid.
34:40One possibility for the origin of the iron may be a cosmic event.
34:46It may actually be meteoric in origin.
34:50And to give you some idea of meteorites such as this one,
34:56I'm going to take the same magnet.
35:01And you can see the magnetism.
35:05There is so much iron in meteorites that it actually holds the magnet.
35:18This suggests that there was some type of catastrophic explosion,
35:23one which not only deposited meteoric iron,
35:27but one that was also intense in temperature and pressure.
35:35An asteroid strike meant North America's mega mammals were doomed.
35:40They couldn't adapt to the challenging conditions which followed the disaster.
35:47But humans could, and the survivors flourished.
35:56It's a controversial theory.
36:02But it wouldn't be the first time that death had come from space.
36:0865 million years ago, an asteroid wiped out the dinosaurs,
36:13leaving behind a giant crater.
36:19The problem is, no crater exists for the Clovis event.
36:26But one man has a theory which might explain why there isn't one.
36:34Planetary geologist Peter Schultz has come to this NASA research center in California
36:40to conduct an experiment with this giant gun.
36:50It's so powerful it can fire projectiles at over 15 times the speed of sound.
36:58This is one of the big guns, the fastest gun in the West.
37:02This is where we have a chance to actually fire small bullets,
37:06small BBs at very high speed.
37:10Schultz and his team will be firing the gun to find out if an object from space
37:15could strike the Earth without leaving a crater.
37:19Schultz is testing a theory that glaciers could have protected the Earth's surface.
37:26During the Clovis era, much of North America was covered
37:30by a giant ice sheet up to a mile deep, a remnant of the last big freeze.
37:38Schultz hopes a scaled-down experiment will show whether glaciers
37:42could have prevented an asteroid from leaving a crater in the underlying rock.
37:48The question we really want to address is,
37:51will the ice actually protect the Earth below?
37:55This is our projectile, it's just an eighth of an inch, about three millimeters or so,
37:59and we're going to be firing this at a speed of about five kilometers per second.
38:07The team prepares the gun for firing.
38:12A number of ultra-high-speed cameras will film the impact for later analysis.
38:21Inside the impact chamber, Schultz prepares the target.
38:26The red sand represents the surface of the Earth.
38:30Let's add some color, at least to the surface layer,
38:34and this way we can see what's going on.
38:37So, we have loose sand underneath, and we have the red layer on top.
38:43We're going to do one experiment when we slam just into this target like we have it,
38:48and the other one where we put a thin layer of ice,
38:51and the idea behind that is whether or not this ice will act as a flak jacket.
38:56Now we just have to go ahead and fire.
39:00The gun is raised into the firing position,
39:04and the countdown begins.
39:10The team waits for the target to hit the target.
39:14The target is in the firing position,
39:18and the countdown begins.
39:22The target is in the firing position,
39:26and the countdown begins.
39:30The team waits in a sealed bunker well away from the gun itself.
39:38Oh!
39:40Now that, that did some damage.
39:43So, this is big.
39:45So, we've got to slow this up now and take a look at the slow-mo.
39:50Kapow!
39:53This is now the entire impact with the streak through and the impact,
39:57stuff that's going downrange at extremely high speed.
40:01That clears away, and we have the crater forming,
40:05and now the crater just grows and grows and grows and grows and keeps growing.
40:13High-speed footage shows the devastating impact on the exposed sand surface.
40:24But the best evidence is inside the impact chamber itself.
40:31Sweet.
40:33Oh, that's nice.
40:36Now, now that did some damage.
40:40So, this, this impact was a, was a good-sized impact.
40:44This was hypervelocity. It slammed in.
40:47It excavated stuff from below.
40:50If we scaled this up to a big crater on the Earth, it would last for millions of years.
40:56So, the next stage is to repair this target,
40:59make it look like it was before we had the impact,
41:02but this time, let's put down a slab of ice,
41:05kind of resembling what might have been on the Earth when there were glaciers.
41:15Now we have the ice on top of the target,
41:18and what we want to know is whether or not this ice actually buffers
41:23or protects the underlying target from the impact.
41:34Sweet.
41:36And we see that the vapor expands,
41:39and we're seeing a little bit of ice come out,
41:42and the ice clears away,
41:45and the real question, I'm really anxious to see,
41:48is whether or not we really produced a crater.
41:51Right now, I don't see a crater.
41:54Let's see what we did.
41:59Oh, man.
42:02Oh, that's remarkable.
42:04The ice was here, and it really protected the target underneath,
42:09and that's just simply loose sand.
42:12So, with time, these pieces disappear, they melt away,
42:17and all we have is a tiny little crater.
42:20And if this were the Earth, it could be easily eroded away.
42:24So, when that ice disappears, there's just nothing left.
42:28It's the perfect crime.
42:36It's only a scale model,
42:38but it shows how an ice sheet could have masked evidence
42:41of an impact 13,000 years ago.
42:48Maybe the megamammals were wiped out by cosmic catastrophe.
42:55One day, we may face a similar disaster.
43:02Advanced warning will be essential for our survival.
43:08Something astronomers in Arizona are working to provide.
43:13This is Mount Lemmon Station, part of the Stewart Observatory.
43:18Here, asteroid hunter Ed Beshore combs space for near-Earth objects, NEOs.
43:26There are millions up there right now,
43:28and not surprisingly, governments worldwide consider them a real threat.
43:35Well, the Earth traveling around the sun is much like a race car
43:39traveling around a circular track,
43:41and a NEO collision might be very much like a car coming suddenly out of the pits
43:46in front of the race cars, representing an immediate impact threat,
43:50and, of course, the consequences of a collision would be devastating.
43:54Each night, Beshore's team photographed the skies,
43:57searching for anything that moves.
44:01Hey, Andrea, have a look at this.
44:04It's really fast.
44:06Yeah, it's quite bright. It's 19th magnitude, and it's got a digest score of 100.
44:10Let's check if it is known.
44:12Yeah. There's no ID on this. This object is new.
44:17The team have found an NEO.
44:21It's painstaking, but vital work.
44:24We take four images spaced over about 45 minutes, about 10 minutes apart.
44:28So here you see four images being shown in sequence.
44:32Our computers register the images so that the stars don't move,
44:36but any object which is moving on the sky is revealed like you see the object here.
44:43Fortunately, this near-Earth object is probably harmless.
44:48This object is, in fact, what's called a virtual impactor,
44:51which means that there is a small, a very small probability
44:54that there might be an impact in the future.
44:58It's big, but luckily it poses little risk.
45:02But for every large object in space,
45:05there are many thousands of smaller ones,
45:08and they can pose a real threat.
45:12Asteroids are like gravel.
45:14If you pick up a handful of gravel,
45:16you're going to find that there's a few large objects in there,
45:19but there's a whole lot more smaller objects.
45:22And it may be these smaller objects that, in fact,
45:25might be on a collision course with the Earth.
45:33And you don't have to look far to see what even a small asteroid can do.
45:42This is Meteor Crater in Arizona.
45:4650,000 years ago, this impact devastated hundreds of square miles.
45:53And the asteroid that did it was just 150 feet across.
46:01But as Russian scientists discovered,
46:03even objects much smaller than this can be lethal.
46:10These images record a scientific expedition
46:13to the remote Tunguska region of Siberia.
46:18This team was sent to investigate reports
46:21of a large explosion in the region in 1908.
46:26What they discovered shocked the scientific world.
46:33The forest was obliterated.
46:37Fallen trees fanned out from a central blast point for hundreds of miles.
46:44There could be only one possible cause.
46:47An asteroid exploding with the power of a nuclear bomb.
46:54Its estimated size to some 30 feet across.
47:07Tunguska is the only hard evidence we have
47:11of a recent impact on planet Earth.
47:15So we can look at that and say, that's pretty scary.
47:18If that was a city underneath there, it would be completely obliterated.
47:22And it's quite interesting that if you look at the area that was destroyed
47:25and superimpose it on London, for example,
47:27virtually the entire area of Greater London would be wiped out.
47:32Tunguska-sized projectiles strike Earth roughly once every century.
47:37Fortunately, the chances of a city being hit are low.
47:42Most of our planet is wilderness or ocean.
47:48But Tunguska hints at our vulnerability.
47:53A deadly asteroid could turn up at any time.
47:59So far, we humans have been lucky.
48:06But that's not to say we'll be lucky in the future.
48:12One day something will happen that will drive humans to extinction.
48:19Our 8,000-year-old civilization
48:22is the product of an immense period of environmental stability.
48:30We humans have experienced catastrophes,
48:33but not yet on a global scale.
48:36If that had happened, we wouldn't be here today.
49:06Transcription by ESO. Translation by —