NARRATOR (SINÉAD CUSACK): Yellowstone is America's first and most famous National Park. Every year over three million tourists visit this stunning wilderness, but beneath its hot springs and lush forests lies a monster of which the public is ignorant.

PROF ROBERT CHRISTIANSEN (US Geological Survey): Millions of people come to Yellowstone every year to see the marvellous scenery and the wildlife and all and yet it's clear that, that very few of them really understand that they're here on a sleeping giant.

NARRATOR: If this giant were to stir, the United States would be devastated and the world would be plunged into a catastrophe which could push humanity to the brink of extinction.

PROF ROBERT SMITH (University of Utah): It would be extremely devastating on a scale that we've probably never even thought about.

PROF BILL McGUIRE (Benfield Greig Centre, UCL): It would mean absolute catastrophe for North America and the problem is we know so little about these phenomena.

NARRATOR: In 1971 heavy rain fell across much of east Nebraska. In the summer palaeontologist Mike Voorhies travelled to the farmland around the mid-west town of Orchard. What he was to discover exceeded his wildest dreams.

PROF MIKE VOORHIES (University of Nebraska): Well I was walking up this gully looking for fossils, the way I'd walked up a thousand gullies before, keeping my eye on the ground looking for pieces of fossils that might have washed down in the rain the previous night and I scrambled up to the top and I saw something that completely astounded me, a sight that no palaeontologist has ever seen.

NARRATOR: It was a sight of sudden, prehistoric disaster. Voorhies's digging revealed the bones of 200 fossilised rhinos, together with the prehistoric skeletons of camels and lizards, horses and turtles. Dating showed they had all died abruptly 10 million years ago.

MIKE VOORHIES: It suddenly dawned on me that this was a scene of a mass catastrophe of a type that I'd never, never encountered before.

NARRATOR: The cause of death, however, remained a mystery. It was not from old age.

MIKE VOORHIES: I could tell by looking at the teeth that these animals had died in their prime. What was astounding was that here were young mothers and their, and their babies, big bull rhinos in the prime of life and here they were dead for no, no apparent reason.

NARRATOR: For the animals at Orchard death had come suddenly. There was another strange feature to the skeletons, an oddity which offered a crucial clue about the cause of the catastrophe.

MIKE VOORHIES: We saw that all of these skeletons were covered with very peculiar growth, soft material that I first thought was a mineral deposit. Then we noticed that it was cellular. It's biological in origin so there was something actually growing on those bones. I had no idea what that stuff was, never seen anything like it.

NARRATOR: A palaeo-pathologist, Karl Reinhard, was sent a sample of the bones.

PROF KARL REINHARD (University of Nebraska): This specimen is typical of the rhino bones. You see this material, in this case it's a whitish material that's deposited on the surface of the original bone. This is peculiar to me, but as I thought back in my experience I realised that this was similar to something that turns up in the veterinary world, a disease called Marie's disease.

NARRATOR: Marie's is a symptom of deadly lung disease. Every animal at Orchard seemed to be infected.

KARL REINHARD: One of the clues was that all of the animals had it. Now that is a very important observation for all the diseases, all the animals to exhibit this disease there had to be some universal problem.

NARRATOR: Scientists discovered the universal problem was ash. 10 million years ago ash had choked them to death.

KARL REINHARD: It may have been a bit like pneumonia with the lungs filling with fluid, except in this case the fluid would have been blood for the ash is very sharp. There'd be microscopic shards of ash lacerating the lung tissue and, and causing the bleeding. I would imagine these animals as stumbling around the thick ash, spitting up blood through their mouths and gradually dying in a most miserable way.

NARRATOR: Only a volcano could have produced so much ash, yet the wide flat plains of Nebraska have no volcanoes.

MIKE VOORHIES: I remember some of my students and I sitting around after a day's digging and just speculating where did this stuff come from? There, there are no volcanoes in Nebraska now. As far as we know there never have been. We, we obviously had to have volcano somewhere that, that produced enough ash to completely drown the landscape here, but where that was really was anybody's guess.

NARRATOR: One geologist in Idaho realised there had been a volcanic eruption which coincided with the disaster at Orchard 10 million years ago, but the site was halfway across North America.

PROF BILL BONNICHSEN (Idaho Geological Survey): It seemed like a really fascinating story which made me think, because I had been working on volcanic rocks in south-western Idaho that potentially could make lots of ash and, and there was some age dates on that that were around 10 million years and I began to wonder wow, could this situation in Nebraska have really been caused by some of these large eruptions that evidently had happened in south-western Idaho.

NARRATOR: The extinct volcanic area, Bruneau Jarbridge, was 1600 kilometres away, a vast distance. How could this eruption have blasted so much ash so far? Bonnichsen was sceptical.

BILL BONNICHSEN: Volcanoes will spew ash for a few tens or maybe a few hundreds of miles. This ash, and it's like two metres thick, in Nebraska is 1600 kilometres or more away from its potential source, so that's an amazing thing. There really had been no previous documentation, to my knowledge, of phenomenon like that.

NARRATOR: Despite his doubts Bonnichsen decided to compare the chemical content of ash from the two sites. He analysed samples from both Bruneau Jarbridge and Orchard and plotted their mineral composition on a graph looking for similarities.

BILL BONNICHSEN: if you have a group of rocks that are very similar to one another they should be a closely spaced cluster of pods. We had these analyses come out from the Orchard site and I thought I'd try the clock again and see how close they were to one another. By golly, they fall right in the same little trend as the Bruneau Jarbridge samples.

NARRATOR: Bonnichsen's hunch had proved correct. Bruneau Jarbridge was responsible for the catastrophe at Orchard. An eruption covering half of North America with two metres of ash was hundreds of times more powerful than any normal volcano. It seemed almost unbelievable, but then Bruneau Jarbridge was that rarest of phenomena which scientists barely understand and the public knows nothing about: a supervolcano.

ROBERT SMITH: Supervolcanoes are eruptions and explosions of catastrophic proportions.

BILL McGUIRE: When you actually sit down and think about these things they are absolutely apocalyptic in scale.

PROF MICHAEL RAMPINO (New York University): It's difficult to conceive of a, of an eruption this big.

NARRATOR: Scientists have never witnessed a supervolcanic eruption, but they can calculate how vast they are.

BILL McGUIRE: Super eruptions are often called VEI8 and this means that they sit at point 8 on what's known as a volcano explosivity index. Now this runs from zero up to 8. It's actually a measure of the violence of a volcanic eruption and each point on it represents an eruption 10 times more powerful than the previous one, so if we take Mount St. Helens, for example, which is a VEI5, we can represent that eruption by a cube of this sort of size, this represents here the amount of material ejected during that eruption. If you go up step higher and look at a VI6, something of the Santorini size for example, then we can represent the amount of material ejected in Santorini by a cube of this sort of size, but if we go up to VEI8 eruptions then we're dealing with something on an altogether different scale, a colossal eruption and you can represent a VI8, some of the biggest VI8 eruptions by a cube of this, this sort of size. It's absolutely enormous.

NARRATOR: Normal volcanoes are formed by a column of magma, molten rock, rising from deep within the Earth, erupting on the surface and hardening in layers down the sides. This forms the familiar dome or cone-shaped mountains.

BILL McGUIRE: Most people's idea of a volcano is a lovely symmetrical cone and this involves magma coming up, reaching the surface, being extruded either as lava or as explosive eruptions as, as ash and these layers of ash and lava gradually accumulate until you're left with a, a classic cone shape.

NARRATOR: Vulcanologists know this smooth flowing magma contains huge quantities of volcanic gases, like carbon dioxide and sulphur dioxide. Because this magma is so liquid these gases bubble to the surface, easily escaping. There are thousands of these normal volcanoes throughout the world. Around 50 erupt every year, but supervolcanoes are very different in almost every way.

First, they look different. Rather than being volcanic mountains, supervolcanoes form depressions in the ground. Despite never having seen a supervolcano erupt, by studying the surrounding rock scientists have pieced together how supervolcanoes are formed. Like normal volcanoes they begin when a column of magma rises from deep within the Earth. Under certain conditions, rather than breaking through the surface, the magma pools and melts the Earth's crust turning the rock itself into more thick magma.

Scientists don't know why, but in the case of supervolcanoes a vast reservoir of molten rock eventually forms. The magma here is so thick and viscous that it traps the volcanic gases building up colossal pressures over thousands of years. When the magma chamber eventually does erupt its blast is hundreds of times more powerful than normal draining the underground reservoir. This causes the roof of this chamber to collapse forming an enormous crater. All supervolcano eruptions form these subsided craters. They are called calderas.

BILL McGUIRE: The main factor governing the size of eruptions is really the amount of available magma. If you've accumulated an enormous volume of magma in the crust then you have at least a potential for a very, very large eruption.

NARRATOR: The exact geological conditions needed to create a vast magma chamber exist in very few places, so there are only a handful of supervolcanoes in the world. The last one to erupt was Toba 74,000 years ago. No modern human has ever witnessed an eruption. We're not even sure where all the supervolcanoes are. Yellowstone National Park, North America. Ever since people began to explore Yellowstone the area was known to be hydrothermal. It was assumed these hot springs and geysers were perfectly harmless, but all that was to change.

ROBERT CHRISTIANSEN: I first came to Yellowstone in the mid-1960s to be a part of a major restudy of the geology of Yellowstone National Park, but at that point I had no idea of what we were to find.

NARRATOR: When geologist Bob Christiansen first began examining Yellowstone rocks he noticed many were made of compacted ash. But he could see no extinct volcano or caldera crater, there was no give-away depression.

ROBERT CHRISTIANSEN: We realised that Yellowstone had been an ancient volcanic system. We suspected that it had been a caldera volcano, but we didn't know where the caldera was or specifically how large it was.

NARRATOR: As he searched throughout the Park looking for the volcanic caldera Christiansen began to wonder if he was mistaken. Then he had a stroke of luck. NASA decided to survey Yellowstone from the air. The Space Agency had designed infrared photography equipment for the moon shot and wanted to test it over the Earth. NASA's test flight took the most revealing photographs of Yellowstone ever seen.

ROBERT CHRISTIANSEN: What was so exciting about looking at the remote sensing imagery was the sense that showed it in one, one sweeping view of what this truly was.

NARRATOR: Christiansen hadn't been able to see the ancient caldera from the ground because it was so huge. It encompassed almost the entire Park.

ROBERT CHRISTIANSEN: An enormous feature. 70 kilometres across, 30 kilometres wide. This had been a colossal supervolcano. Certainly one of the largest known anywhere on earth.

NARRATOR: Bob Christiansen was determined to find out when Yellowstone had last erupted. He began examining the sheets of hardened ash, dozens of metres thick blasted from the ground during the eruption. What he found was 3 separate layers. This meant there had been 3 different eruptions. When Christiansen and his team dated the Yellowstone ash he found something unexpected. The oldest caldera was formed by a vast eruption 2 million years ago. The second eruption was 1.2 million years old and when he dated the third and most recent eruption he found it occurred just 600,000 years ago. The eruptions were regularly spaced.

ROBERT CHRISTIANSEN: Quite amazingly we realised that there was a cycle of caldera-forming eruptions, these huge volcanic eruptions about every 600,000 years.

NARRATOR: Yellowstone was on a 600,000 year cycle and the last eruption was just 600,000 years ago. Yet there was no evidence of volcanic activity now. The volcano seemed extinct. That reassuring thought was about to change. There was another geologist who was fascinated by Yellowstone's volcanic history. Like Bob Christiansen, Professor Bob Smith has been studying the Park for much of his career. In 1973 he was doing field work, camping at one end of Yellowstone Lake.

ROBERT SMITH: I was working at the south end of this lake at a place called Peal Island. I was standing on the island one day and I noticed a couple of unusual things. The, the boat dock that we normally would use at this place seemed to be underwater. That evening as I was looking over the expanse of the south end of the lake I could see trees that were being inundated by water. I took a look at these trees and they were be, being inundated with water a few inches, maybe a foot deep and it was very unusual for me to see that because nowhere else in the lake would the lake level have really changed. What did it mean? We did not know.

NARRATOR: Smith commissioned a survey to try to find out what was happening at Yellowstone. The Park had last been surveyed in the 1920s when the elevation, the height above sea-level, was measured at various points across Yellowstone. 50 years later, Smith surveyed the same points.

ROBERT SMITH: The idea was to survey their elevations and to compare the elevations in the mid-70s to what they were in 1923 and the type of thing that we did is to make recordings at a precision level of, of a few millimetres.

NARRATOR: The two sets of figures should have been similar, but as the survey team moved across the Park, they noticed something unexpected: the ground seemed to be heaving upwards.

ROBERT SMITH: The surveyor said to me there's something wrong and he said it's not me, it's got to be something else, so we went through all the measurements again trying to be very careful and the conclusion kind of hit me in the face and said this caldera has uplifted at that time 740 millimetres in the middle of the caldera.

NARRATOR: As the measuring continued, an explanation for the submerged trees began to emerge. The ground beneath the north of Yellowstone was bulging up, tilting the rest of the Park downwards. This was tipping out the sound end of the lake inundating the shoreside trees with water. The vulcanologist realised only one thing could make the Earth heave in this way: a vast living magma chamber. The Yellowstone supervolcano was alive and if the calculations of the cycle were correct, the next eruption was already overdue.

ROBERT CHRISTIANSEN: Well this gave us a real shiver of nervousness if you will about the fact that we have been through this 600,000 year cycle and that the last eruption was about 600,000 years ago.

ROBERT SMITH: I felt like telling people, that is we basically have on our hands a giant.

NARRATOR: The scientists had found the largest single active volcanic system yet discovered. There were many things they needed to find out. How big was the magma chamber deep underground, how widespread would the effects of an eruption be and crucially, when would it happen? To answer any of these questions vulcanologists knew they first had to understand Yellowstone's mysterious magma chamber.

ROBERT SMITH: It's incredibly important to understand what's happening inside of the magma chamber because that pressure and that heat, the fluid is what's triggering the final eruption. It's like understanding the primer in a bullet.

NARRATOR: Understanding the magma chamber would be very difficult. Smith and his team needed to discover the size of something 8 kilometres below the ground. They began harnessing information from an ingenious source: earthquakes.

ROBERT SMITH: Well, what we have here is a seismometer. This is the working end of a seismograph, the device that's used to record earthquakes. It is able to pick up the smallest of earthquakes in, in Yellowstone plus it picks up moderate to large earthquakes around the world, it is so sensitive. This forms one of a network of 22 seismograph stations in Yellowstone that is used for monitoring and all the data are transmitted to a central recording facility at the University of Utah.

NARRATOR: Like many thermal areas, Yellowstone has hundreds of tiny earth tremors each year. They are harmless, but in his seismographic lab Smith has been using them to trace the size of the magma chamber.

ROBERT SMITH: Earthquakes are essentially telling you the pulse. They tell you the real time pulse of how the caldera is deforming, of how faults are fracturing.

NARRATOR: Bob Smith's 22 permanent seismographs are spread across the Park. They detect the sound-waves which come from earthquakes deep underground. These waves travel at different speeds depending on the texture of what they pass through. Soundwaves passing through solid rock go faster than those travelling through molten rock or magma. By measuring the time they take to reach the seismographs Smith can tell what they've passed through. Eventually this builds up a picture of what lies beneath the Park.

ROBERT SMITH: The magma chamber we found extends basically beneath the entire caldera. It's maybe 40-50 kilometres long, maybe 20 kilometres wide and it has a thickness of about 10 kilometres. So it's a giant in volume and essentially encompasses a half or a third of the area beneath Yellowstone National Park. NARRATOR: The magma chamber was enormous. If it erupted it would be devastating. To discover the extent of the devastation scientists had to understand the force of the eruption. The clues to this could be found in a much smaller volcano halfway across the world: the Greek island of Santorini. The eruption here 3,500 years ago, although not VEI8 in scale, did have a small magma chamber. Professor Steve Sparks has spent much of his career studying Santorini.

PROF STEVE SPARKS (University of Bristol): When I first came to Santorini and started to look at the pumice deposits from these caldera forming eruptions I found evidence of a dramatic change in the power and violence of the eruption.

NARRATION: By examining the layers of Santorini pumice Sparks discovered magma chambers could erupt with almost unimaginable force and spread their devastation widely.

STEVE SPARKS: There's dramatic evidence of a sudden increase in the power. Huge blocks about 2 metres in diameter were hurled out of the volcano reaching 7 kilometres and smashing into the ground and to do that the blocks must have been thrown from the volcano at hundreds of metres per second, about the speed of Concorde and you can imagine this enormous red rock crashing in and breaking up on impact.

NARRATOR: To understand why caldera volcanoes could erupt with such power Sparks replicated their violence at one trillionth of the scale.

STEVE SPARKS: OK, so we need this…

NARRATOR: In the lab he modelled a reaction which occurs in the magma chamber of an erupting caldera.

STEVE SPARKS: The problem is we can't go into a magma chamber so the next best thing to do is to go to the laboratory and try and simulate what happens in the magma chamber and in the pathway to the surface.

NARRATOR: Sparks believed escaping volcanic gas trapped in the magma might be responsible for the violence of the eruptions. Into a glass flask - the magma chamber - he poured a mixture of pine resin and acetone. the pine resin mimicked the magma, the acetone modelled trapped volcanic gases like carbon dioxide and sulphur dioxide.

STEVE SPARKS: Pine resin is a very sticky, stiff material so it has some properties which are rather like magma and we thought that if we could get a, a gas which dissolved in pine resin, like acetone, then we could get a, a laboratory system which would represent the, the natural case.

NARRATOR: Sparks then created a vacuum above the flask to mimic the depressurisation that occurs in the magma chamber when a supervolcano begins its eruption and the dissolved volcanic gas can expand. When the vacuum reached the liquid it caused a dramatic change. The dissolved acetone suddenly became a gas. This made the resin expand causing violent frothing and blasting the contents out of the chamber.

STEVE SPARKS: These experiments give us tremendous insight into the tremendous power of gases coming out of solution and enabled to drive these very dramatic explosive flows.

NARRATOR: Unlike supervolcanoes, normal volcanoes don't have this vast reservoir of magma and trapped volcanic gases and don't have the potential for such powerful eruptions. But experiments in the laboratory cannot answer the biggest question of all surrounding Yellowstone: when will it next erupt? Scientists face a problem. They have never seen a supervolcano erupt. Until a VEI8 eruption is observed and analysed no-one knows what the telltale precursors would be to a Yellowstone eruption.

BILL McGUIRE: We can actually model volcanoes and their activity. We can do it in the laboratory on computer, but we need observational data in order to make those models realistic.

ROBERT SMITH: What the precursors might be for a giant volcanic eruptions they've never been observed scientifically and they've never been documented, so we don't know what to look for.

ROBERT CHRISTIANSEN: Nobody wants to see a global disaster of course and yet we'll never really fully understand the processes involved in these supervolcanic eruptions until one of them happens.

NARRATOR: A terrible truth underlies all mankind's efforts to understand the vast mechanisms which drive VEI8 eruptions. Ultimately trying to find out what makes supervolcanoes work may be pointless. Consider the last one. 74,000 years ago a supervolcano erupted here in Sumatra. It would have been the loudest noise ever heard by man. It would have blasted vast clouds of ash across the world.

The resultant caldera formed Lake Toba, 100 kilometres long, 60 kilometres wide. it was, in short, colossal. Scientists are only now beginning to understand the effects of so much ash on the planet's climate. This is the ocean core repository at Columbia University in America. It contains thousands of drill samples from seabeds round the world, a historical keyhole through which scientists, like Michael Rampino can view volcanic history.

MICHAEL RAMPINO: The size of the Toba eruption was enormous. We're talking about, about 3,000 cubic kilometres of material coming out of that volcano. That's about 10,000 times the size of the 1980 Mount St. Helens eruption which people think of as a large eruption, a truly super eruption.

This is an ocean drilling core from the central Indian Ocean. It's about 2,500 kilometres from the Toba volcano and here are 35 centimetres of ash deposited after the Toba eruption. It shows that this Toba eruption was a supervolcanic event, it was much, much bigger than any other volcanic eruption we see in the geological record. Chemical analysis of the ash tells us that this eruption was rich in sulphur, would have released a tremendous amount of sulphur dioxide and other gases into the stratosphere which would have turned into sulphuric acid aerosols and affected the climate of the Earth for years.

NARRATOR: For a long time scientists have known that volcanic ash can affect the global climate. The fine ash and sulphur dioxide blasted into the stratosphere reflects solar radiation back into space and stops sunlight reaching the planet. This has a cooling effect on the Earth. In the year following the 1991 eruption of Mount Pinatubo for instance the average global temperature fell by half a degree Celsius. By comparing the amount of ash ejected by past volcanoes with their effect on the Earth's temperature, Rampino has estimated the impact of the Toba eruption on the global climate 74,000 years ago.

MICHAEL RAMPINO: I'm plotting a simple graph where one side there's sulphur released in millions of tons by volcanic eruptions and on the other side there's a cooling in degree Celsius that we saw after these volcanic eruptions. I'm plotting as points the historical eruptions like Mount St. Helens, Krakatoa, Pinatubo, Tambora. There's a nice correlation between the sulphur released into the atmosphere and the cooling.

NARRATOR: Because of this relationship between the sulphur released by large volcanoes and global cooling, Rampino can calculate the drop in temperature caused by the Toba eruption.

MICHAEL RAMPINO: We can see this kind of plot predicts that the Toba eruption was so large that the temperature change after Toba in degrees Celsius would have been about a 5 degree global temperature drop, very significant, very severe global cooling. NARRATOR: Five degrees Celsius average drop in global temperature would have been devastating causing Europe's summers to freeze and triggering a volcanic winter.

MICHAEL RAMPINO: Five degrees globally would translate into 15 degrees or so of summer cooling in the temperate to high latitudes. The effects on agriculture, on the growth of plants, on life in the oceans would be catastrophic.

NARRATOR: This global catastrophe would have continued for years, dramatically affecting life on Earth, but what impact did it have on humans? The answer may be buried not inside the ancient rocks, but deep within us all. Lynn Jorde and Henry Harpending are scientists specialising in human genetics. Since the early 1990s they have been studying mitochondrial DNA using the information to investigate mankind's past. Most of our genetic information is stored in the nuclei of our cells, but a small, separate quantity exists in another component, the part which produces the cells' energy, the mitochondria.

PROF LYNN JORDE (University of Utah): Mitochondria have their own genes. It's a small number of genes, a small amount of DNA, but it's distinct from the rest of the DNA in the cell and because of the way mitochondria are transmitted from one generation to the next, they're, they're inherited only from the mother so they give us a record of the maternal lineage of a population.

NARRATOR: Mitochondrial DNA is inherited only by the mother. All mutations are passed on from mother to child, generation after generation at a regular rate. Over time, the number of these mutations accumulate in a population.

LYNN JORDE: Every event that takes place in our past, every major event, a population increase, a population decrease, or the exchange of people from one population to another changes the composition of the mitochondrial DNA in that population, so what happens is that we have a record of our past written in our mitochondrial genes.

NARRATOR: By knowing the rate of mutation of mitochondrial DNA and by a complex analysis of the distribution of these mutations, the geneticists can estimate the size of populations in the past. Several years ago they began seeing a strange pattern in their results.

LYNN JORDE: We expected that we would see a pattern consistent with a relatively constant population size. Instead, we saw something that departed dramatically from that expectation. We saw a pattern much more consistent with a dramatic reduction in population size at some point in our past.

NARRATOR: This confirmed what other geneticists have noticed. Given the length of time humans have existed, there should be a wide range of genetic variation, yet DNA from people throughout the world is surprisingly similar. What could have caused this? The answer is a dramatic reduction of the population some time in the past: a bottleneck.

LYNN JORDE: We imagine the population diagrammed like this. In the distant past back here we have a large population, then a bottleneck looking like this and then a subsequent enlargement of population size again, so we would have families of people in the distant past with a significant amount of genetic diversity, but when the bottleneck occurs, when there's a reduction in population size perhaps only a few of those families would survive the bottleneck.

We have a dramatic reduction in genetic diversity during this time when the population is very small and then after the bottleneck the people who would we, who we would see today would be descendants only of those who survived, so they're going to be genetically much more similar to one another reducing the amount of genetic variation.

NARRATOR: Human DNA is so similar the scientists concluded the population reduction had been catastrophic. PROF HENRY HARPENDING (University of Utah): It seemed so incredible, you know the idea that all of us, now there's 6 billion people on Earth, and what the data were telling us was that we, you know our species was reduced to, you know, a few thousand. Suddenly it hit us, we had something to say about human history.

LYNN JORDE: Our population may have been in such a precarious position that only a few thousand of us may have been alive on the whole face of the Earth at one point in time, that we almost went extinct, that some event was so catastrophic as to nearly cause our species to cease to exist completely.

NARRATOR: It is an astonishing revelation, but the key was to find out when and why it happened. Because mitochondrial DNA mutates at an average rate these scientists believe, controversially, that they can narrow down the date of the bottleneck.

LYNN JORDE: Mutations in the mitochondria take place with clocklike regularly, so the number of mutations give us a clock essentially that we can use to approximately date the major event. In the case of a population bottleneck we think that this would have occurred roughly 70-80,000 years ago, give or take some number of thousands of years. So then the real question is: what could have caused such a reduction, an extreme reduction, in the human population down to as few as 5 or 10,000 individuals?

NARRATOR: As for what caused this dramatic reduction in population the geneticists had no idea. Henry Harpending began touring universities to talk about the bottleneck. He was invited by anthropologist Stanley Ambrose to give a lecture to his students.

HENRY HARPENDING: Well Stanley is full of ideas, he's the kind of scientist that plucks things from all over and puts them together.

PROF STANLEY AMBROSE (University of Illinois): I sat in on the lecture and he start4ed talking about this human population bottleneck and I thought what could have caused it and at that point I broke out into a sweat. I went up to Henry and said I've just read a paper, and it's on the top of my desk now, that may have an explanation for why this population bottleneck occurred.

HENRY HARPENDING: I didn't read it till a week later and when I read it you know it was like somebody kicking you in the face. There it was.

STANLEY AMBROSE: The paper was about the super eruption of a volcano called Toba in Sumatra.

NARRATOR: This team of scientists believe the bottleneck occurred between 70 and 80,000 years ago, although this date is hotly debated. Toba erupted in the middle of this period, 74,000 years ago. If there really is a connection this research has terrifying implications for a future Yellowstone eruption. It could well be of a similar size and ferocity to Toba. Like Toba, it would have a devastating impact, not just on the surrounding region, North America, but on the whole world.

MICHAEL RAMPINO: If Yellowstone goes off again, and it will, it'll be disastrous for the United States and eventually for the whole world.

NARRATOR: Vulcanologists believe it would all start with the magma chamber becoming unstable.

BILL McGUIRE: You'd start seeing bigger earthquakes, you may see parts of Yellowstone uplifting as magma intrudes and gets nearer and nearer the surface.

ROBERT SMITH: And maybe an earthquake sends a rupture through the brittle layer, you've broken the lid of the pressure cooker.

BILL McGUIRE: This would generate sheets of magma which will be probably rising up to 30, 40, 50 kilometres sending gigantic amounts of debris into the atmosphere.

ROBERT CHRISTIANSEN: Where we are right now would be gone. We would be instantly incinerated.

MICHAEL RAMPINO: Pyroclastic flows will cover that whole region, maybe kill tens of thousands of people in the surrounding area.

BILL McGUIRE: You're getting a, an eruption which we can barely imagine. We've never seen this sort of thing. You wouldn't be able to get within 1,000 kilometres of it when it was going like this.

ROBERT CHRISTIANSEN: The ash carried in the atmosphere and deposited over large areas of the United States, particularly over the great plains, would have devastating effects.

BILL McGUIRE: The area that would be affected is, is the bread basket of North America in effect and it produces an enormous amount of grain on a global scale really. That's, that's, that's the problem and you would see nothing. The harvest would vanish virtually overnight.

ROBERT CHRISTIANSEN: All basic economic activity would certainly be impacted by this and let alone changes in the climate that could possibly be induced.

MICHAEL RAMPINO: The climatic effects globally from that eruption will be produced by the plume of material that goes up into the atmosphere. That'll spread worldwide and will have a cooling effect that will probably knock out the growing season on a global basis. We can't really overstate the effect of these huge eruptions. Civilisation will start to creak at the seams in a sense.

ROBERT SMITH: The fact that we haven't seen one in historic time or documented means the human race really is not attuned to these things because they're such a rare event.

MICHAEL RAMPINO: It's really not a question of if it'll go off, it's a question of when because sooner or later one of these large super eruptions will happen.

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