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Killer Lakes
BBC Two 9.00pm Thursday 4 April 2002

Cameroon's Lake Nyos, shortly after the 1986 overturn Click for programme summary
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NARRATOR (MARTIN SHAW): Beneath the peaceful surface of this lake lurks a monster which has the power to wipe out the millions of people who live within its reach because deep in the bottom layers the water holds a sinister secret. It's killed before, it can kill again. Scientists are now in a race against time to stop this serial killer from striking once more. Lake Kivu in Rwanda. Two million people live and work along its shores. Shortly after these pictures were taken the volcano, Mount Nyiragongo, just 18km away, erupted.

NEWSREADER (PETER SISSONS): Thousands of refugees flee the devastation of an African volcano. With up to half a million homeless there are desperate appeals for help.

NEWSREADER (ANNA FORD): The volcano crisis goes from bad to worse. 50 people are feared dead in an explosion.

NARRATOR: On 17 January 2002 the world's cameras recorded the worst human disaster since Rwanda's civil war, but behind the scenes scientists were concerned that an even greater catastrophe was about to overtake the millions who live here.

BILL EVANS (US Geological Survey): People thought that this was the disaster, but ironically these people were lucky. I was watching this and thought this could be just the beginning. Millions could die.

NARRATOR: They feared the eruption and the earthquakes that followed it would unleash the silent killer that hides at the bottom of the lake.

PROF GEORGE KLING (University of Michigan): There's something at the bottom of this lake that's much more terrible and dangerous than any volcano or earthquake. It's something we're just beginning to understand. If the lava had flowed to the bottom of the lake it could have been catastrophic.

NARRATOR: It may sound like science fiction. Thousands have already been killed and if something isn't done many more will perish, struck down by a deadly force that in a modern day version of Pompeii kills every living thing in its path. The killer in the lake first struck in 1984 in Cameroon, West Africa. Early one morning 18 years ago Ahadji Abdou was making the journey to his farm in Cameroon and it was a day he would never forget.

AHADJI ABDOU: I thought it was a car accident. I stopped my bicycle and I stood still. I saw dead people all over the road. I knew something terrible had happened here. I thought I was going to die.

NARRATOR: Within hours 37 people would be found dead. Strange stories of how they had died were reaching the nearby village.

MOTAPON OUMAROU: We heard they were massacred. Two guys came running and told us, "We don't know what it is, but it kills people." They told us there were 12 people in the truck and 10 of them died. We asked them, "How did they die?" They told us we were sitting on the top of the truck, a pick-up. The others were under the cover. The driver was the first to get out and see what was wrong with his engine and he fell. The rest of the men in the back decided to get out and died. You understand?

NARRATOR: It seemed like everyone had died without a struggle. It was as if they had been hit by a highly contagious disease. The first doctor on the scene was Pierre Zambou. Nothing prepared him for what he was about to see.

DR PIERRE ZAMBOU (Foumbot Hospital, Cameroon): People were afraid. We had never seen anything like it before. People lying dead on the road like the coffee we dry on the drying sheds.

NARRATOR: People were terrified the infection would spread.

PIERRE ZAMBOU: We had no masks, nothing, no gloves either. We used our bare hands. We put them in the military jeep and left.

NARRATOR: Death had come so suddenly and to so many there were rumours that someone had released a deadly biological weapon.

PIERRE ZAMBOU: A few minutes later the corpses were at the hospital, all dumped there. The police were there too to prevent all contact with the population.

NARRATOR: If someone was secretly testing a new biological weapon the US government wanted to know about it. A few months later they sent a scientist to investigate the bizarre events of Monoun. That scientist was Professor Haraldur Sigurdsson.

PROF HARALDUR SIGURDSSON (University of Rhode Island): It was a very serene, calm looking place, but you had the sense that there had been death here before. When we walked along the path that the people had been walking along that were killed we had a very strange sensation to, to be in that area. We were charged with the responsibility of trying to solve this mystery, trying to figure out what is going on here, possibly something that has never been seen before, never been documented before.

NARRATOR: Biological warfare was quickly ruled out. It looked much more as if the victims had been suffocated, but by what? Sigurdsson decided to interview eye-witnesses to reconstruct what had happened. Some had caught a glimpse of the killer.

MOTAPON OUMAROU: We saw a cloud, a thick white cloud a few feet away from us. In an instant it was gone.

NARRATOR: This was the first clue. A deadly white cloud and the killer had left a second, apparently more important, clue.

PIERRE ZAMBOU: When we got up there we passed someone who was fleeing who told us you're going to die, you're going to die, something smells bad up there.

NARRATOR: It was the smell of rotten eggs and gunpowder, a strange smelling white cloud. At first he had no idea what it could be, but there was one other clue: all 37 people had died on the road that ran past the lake. Wherever the killer had come from Sigurdsson was now convinced that it must have something to do with the lake, so he decided to venture out onto the water.

HARALDUR SIGURDSSON: But we had really no idea what was in the lake and when we got out on it, on a little boat it was scary. We hadn't really no idea whether the lake was safe to be on and so it was an uneasy feeling.

NARRATOR: At the deepest point of the lake he stopped and took samples from the very bottom. Slowly he pulled them up.

HARALDUR SIGURDSSON: As we were hauling them in and when they were getting closer to the surface we see that they're bursting with gas. Huge bubbles are coming up and then I immediately realised that the deep waters of the lake are saturated with a gas.

NARRATOR: A gas he couldn't see, taste or smell. Sigurdsson suddenly realised there was only one gas it could be, the gas that all of us breathe out, a gas that is harmless in small amounts, but suffocates people in high concentrations.

HARALDUR SIGURDSSON: Carbon dioxide. It was clear to me then that carbon dioxide was the asphyxiating agent, the deadly agent that had let to the disaster in Monoun.

NARRATOR: There was now a real mystery. What could have forced huge quantities of carbon dioxide out of the lake? But before he had time to find an explanation the killer struck again.

NEWSREADER: Hundreds of people have died in a natural disaster in the West African state of Cameroon.

NEWSREADER: A deadly natural gas has left at least 1,200 people dead.

NARRATOR: The tragedy at Lake Nyos two years later in 1986 stunned the world. Another lake in Cameroon was linked to mysterious deaths, but this time the extent of the damage was so vast the news could not be suppressed.

PAPA NYAKO: I went down to Nyos. There was no people, everybody was dead.

NARRATOR: Entire villages were wiped out filled with corpses and dead cattle. There was no sign of panic. People had fallen dead while sleeping or cooking. Again they seemed to have been suffocated along with all their animals.

ZACHEUS YAMAH NGONG: I went up to the health centre. The women that were in the ward they were already dead.

NARRATOR: The death toll was staggering. 1,800 people had died. It was 50 times more deadly than Monoun. There were some survivors, but they woke up to scenes of horrific devastation.

MONICA LOM NGONG: I was sitting, just sitting among the dead people inside the house, some of them were outside, some of them behind the houses and it was animals everywhere lying, cows, dogs, cows, everything, so I was confused by then. All the family, we were 56, but 53 died.

NARRATOR: By the time the emergency services got to the scene there was no sign of the killer, but once again it had left a trail of important clues. This time there were many more witnesses with a lot more evidence.

PAPA NYAKO: The thing was white, white like cloth. It didn't go up in the air, it mostly went down near the ground.

DAVID CHIA WAMBONG: We heard a loud sound just like explosion. I went outside and saw all my cows lying on the ground. I went back inside and saw my wife and daughter had fallen and were lying near a drum of water.

NARRATOR: And just like at Monoun the deadly cloud had left a familiar calling card.

ZACHEUS YAMAH NGONG: There was a smell like gunpowder.

NARRATOR: And there was something else. Those who survived had strange marks all over their body.

MONICA LOM NGONG: When I wake up I had burns on my left arm. By then I was feeling no pains. The arm was giving in a way that it was nearly having rotten because of the wounds.

NARRATOR: Only one thing seemed to account for all this evidence. It would have produced an explosion. It was well known for producing smelly gases and it certainly could cause burns and when scientists looked further it soon made perfect sense. Nyos was no ordinary lake. It was sitting in the mouth of a volcano and it was the same for Monoun. Both these crater lakes were formed on top of a volcanic chain which includes the still active Mount Cameroon .The search for the culprit seemed to be over.

NEWSREADER: The gas came up from a volcano beneath the waters of Lake Nyos in the remote north-west of the country...

NEWSREADER: least 1,200 people are reported dead in Cameroon, the victims of a poisonous gas leak from a volcanic lake...

NEWSREADER (MICHAEL BUERK): ...poisoned by a sudden release of gas from a volcano lying beneath a lake.

NARRATOR: No one could predict when the volcano would erupt again, so the Cameroonians decided to evacuate anyone living close to the lake. Nyos was abandoned, but what no one knew was that new evidence was to be uncovered, evidence which would lead everyone on a journey into the unknown. Among the first on the scene were the Americans. For them what started as an ordinary investigation would end in an extraordinary discovery that would revolutionise our understanding of how lakes kill. George Kling and Bill Evans would begin to understand the most intimate mysteries of the monster deep inside the lake and for the first time find a way to tame it. Their first glimpse of the lake was puzzling and for George Kling who had been to the site just a year before it was something of a shock.

GEORGE KLING: I had a picture in my mind of what the lake was like when I was here about a year earlier, beautiful blue mountain lake and the, the lake was completely transformed. There were mats of floating vegetation strewn across this really muddy, red water and there were dead cattle through the valleys.

NARRATOR: At this stage they were convinced that only a volcano could have caused the catastrophe.

BILL EVANS: When we first arrived everything seemed to point to a volcanic eruption. First there was the fact that the lake occupies a volcanic crater. Second, there was the scale of the disaster and third, there was the burns on some of the victims. All of that seemed to be consistent with a volcanic eruption.

NARRATOR: But as they walked around the top of the crater they realised that if it really was a volcanic eruption it looked like a very strange one indeed.

GEORGE KLING: We walked around looking for any evidence of volcanic activity - lava flows or steam beds, sediment that had been recently thrown up from the lake and really we found none of that. There was no evidence that there was a large volcanic event or eruption that had just occurred and that led us to feel that the next thing was to go out on the lake itself. We were too busy to be frightened. We set out sort of numbed by everything around us and what we were seeing and the idea that so many people had been killed. We just focused on the work. The amount of damage almost put you in shock. You just couldn't believe what you were seeing. This entire cliff here was - looks normal today - but was completely denuded. All the vegetation, all the soil was just washed off and it was really, had to be a tremendous amount of energy to, to do that.

NARRATOR: It looked as if something had produced a massive tidal wave.

BILL EVANS: The damage extended to about 80-100m above the level of the lake, so water had definitely passed over it at least that high, gone down the other side of the promontory and created a large wave in the entire southern end of the lake. It had washed up and on the way back the water brought vegetation, trees, grasses, lots of plant material back into the lake. At the same time, on the other side of the lake there was almost no damage, so the damage was heavily focussed in this corner of the lake suggesting a gas jet out of the lake, which is very consistent with a volcanic eruption from beneath the lake.

NARRATOR: So they carried out the standard tests for signs of a volcanic eruption in the water itself. First, had the temperature of the water changed? The volcano would be so hot that the water at the bottom of the lake should still be warm, very warm. They were expecting a huge increase in temperature, up to 40 degrees, but the temperature of the water was normal. Bill Evans and George Kling were amazed, so they carried out more tests. This time they were looking for sulphur and chloride which are very common in volcanic eruptions, but again the results were very puzzling.

GEORGE KLING: Well, one of the most interesting things was that many of the survivors reported smelling rotten eggs or gunpowder and that's unmistakeable sulphur gas, but when we looked in the water we could not find any sulphate or any chloride, very small amounts, which didn't fit.

NARRATOR: The volcano theory just didn't add up.

GEORGE KLING: It was so strange all of the evidence that we were coming up with was really a lack of evidence for a volcanic hypothesis.

NARRATOR: But if a volcano wasn't responsible for the tragedy what was? What else could have caused the damage around the lake and explain the strange smell and the burns? What exactly was hidden in the depths of the lake? The first clue to the identity of the killer would come from the only other lake that had killed - Lake Monoun - and it would change everything. Back in the US Haraldur Sigurdsson was analysing his samples of carbon dioxide. His aim was to find out where the gas actually came from. Gases have different fingerprints depending on where they come from. Plants produce one kind of carbon, volcanoes another, but this was something else. The chemical fingerprint of this gas showed that it came from deep within the Earth itself. The lethal gas found at the bottom of Lake Monoun did not come from the volcano. Instead Sigurdsson's experiments seemed to show that the carbon dioxide was seeping slowly up through cracks from deep within the Earth. It led him to come up with an amazing new explanation.

HARALDUR SIGURDSSON: The lake is rather like a bottle of soda in a way with the gas coming in, but the gas isn't forming bubbles in the deep water because there is so much weight of water on the gas that it is dissolved inside the water. We don't see the gas. Now of course if you release the pressure you get the gas coming up and this is what happened at Lake Monoun.

NARRATOR: Just like in a bottle of soda the gas dissolved in the lake is under pressure and like a bottle of soda if the pressure was suddenly released the gas could explode out of the top. But in this case the soda bottle was a huge lake. Sigurdsson called his theory Lake Overturn. No one had ever described such a strange event. When Sigurdsson presented his findings it sent shock waves throughout the scientific community.

HARALDUR SIGURDSSON: I realised that the only explanation that was viable was the carbon dioxide burst. That the carbon dioxide was coming out of the deep water. Other scientists wouldn't accept this. Now this was such an outrageous, novel, iconoclastic idea that it was very difficult for them to accept.

NARRATOR: Back at Nyos, the idea that the lake itself had somehow suffocated its victims was still hard for the scientists to come to terms with.

GEORGE KLING: The idea that a lake itself could have done this didn't really fit with anyone's expectation. Lakes don't kill people. People may drown, but they just don't explode and kill people and so it was a little bit of a leap to go from a relatively easy explanation of a volcanic eruption to something that no one had ever heard of before.

NARRATOR: So Bill Evans and George Kling decided to put Sigurdsson's theory to the test. Could there really be carbon dioxide trapped at the bottom of Lake Nyos?

BILL EVANS: See what we've got. Whoa, look at that.

GEORGE KLING: When we brought the first samples up from the bottom and they literally exploded out, out of the sampler because there was so much gas in them we knew that it was the lake itself. It was not a volcano, it was a completely different and new kind of eruption.

NARRATOR: But how could massive amounts of carbon dioxide be getting into the lake? Bill and George started hearing strange stories from the local people, stories of magical springs that killed frogs and birds. As soon as any small animals approached the mysterious springs they dropped down dead, just like the people of Nyos. The scientists decided to go and look for these magical springs and as soon as they found them they knew they were in the right place. There were bubbles of gas in the water. When they tested the water they discovered they were bubbles of carbon dioxide. The amounts of gas were alarming. Could these springs be the very piece of evidence they'd been looking for?

BILL EVANS: You can visualise a spring such as this on the bottom of the lake feeding the lake for 100 years and given a 100 years there's enough build-up of that CO2 to cause the eruption.

NARRATOR: The theory seemed to make sense, but they couldn't be sure until they compared their samples from the springs to those of the lake. The results they got were beyond their expectation. It was an exact match.

GEORGE KLING: We realised that's it. The springs are the source of the gas into the bottom of the lake. One of these soda springs - there may be many of them - were feeding CO2 into the bottom of the lake.

NARRATOR: They'd cracked it and so they formulated their theory. Carbon dioxide springs like these pump vast amounts of CO2 into the water at the bottom of the lake. As more and more carbon dioxide was pumped in the gas concentration rose turning the lake into nothing less than a huge chemical time bomb, just waiting for the one event that would trigger it to explode. But what was that trigger? There was some evidence on the cliffs around Lake Nyos itself. One side of the crater had a strange scar that had appeared at the same time as the disaster. It was the tell-tale signs of a landslide.

BILL EVANS: In 1986 there was evidence of a landslide. Right in that area ahead of you that's shaped sort of like a V. All that material was freshly, had freshly fallen into the lake. Probably hundreds of tons of rock and mud slid down from that area down into the lake.

NARRATOR: But would it have been enough to trigger a disaster on this scale?

BILL EVANS: The lake here gets very steep so it's conceivable that that slide could have continued all the way down to the bottom of the lake delivering energy to deeper layers and starting a kind of a chain reaction, eventually leading to the disaster.

NARRATOR: It seemed they had found the fuse that lit the bomb and turned the lake into a killer. The scientists now had all the clues they needed to explain what happened that night at Lake Nyos and why 1,800 people had died. Just before 9pm the villagers prepared for bed for the last time. It was the night before market, so the village of Nyos was packed with people. At the far corner of the lake the tragic events unfold. A landslide unleashes hundreds of tons of rocks into the lake. They move down and penetrate the top of the layer of water where carbon dioxide has dissolved. Bubbles of carbon dioxide move up to the surface. Water droplets form on the surface of the gas turning it white. Once the crater is full a dense cloud of gas pours over the lip of the spillway bearing down on the villages at over 60km/h. The gas crept over those sleeping in their beds suffocating them in their sleep. There would be no time to scream or cry out. In the village of Nyos all but six of the villagers died. The gas cloud travels on to neighbouring villages. For 1,800 people death is now inevitable, but the deadly process of lake overturn wasn't over yet. The turbulence caused by the landslide is moving down to the bottom where the gas concentration's at its highest. The gassy water explodes upwards in a huge fountain of water and foam killing all the fish and washing over one side of the crater. This was the explosion that some of the survivors heard and thought was a volcano. The huge jet sends a tidal wave 25m high racing across the shore with enough force to strip it of bare rock and dragging vegetation back into the water with it and then the whole process stops. The lake overturn theory seemed to explain everything, except for two outstanding questions: what was the mysterious smell? The most likely explanation is that very high concentrations of carbon dioxide can produce hallucinations where people smell things like rotten eggs and gunpowder, but the bigger puzzle was the strange burns and blisters left on the bodies of the victims. These burns were one of the mysteries to be solved. How had these victims come to be so terribly scarred? When British doctor, Peter Baxter, saw the wounds he was baffled.

DR PETER BAXTER (Institute of Public Health): We were really very puzzled with the distribution of these ulcers and burns on the skin and the fact that they didn't look like normal burns. On one victim who'd died the blisters were all over the body.

NARRATOR: This seemed inexplicable. Lake overturn showed that the villagers were asphyxiated by carbon dioxide, but carbon dioxide doesn't burn. Then Peter Baxter realised that there is a gas very similar to carbon dioxide that does produce blisters - the even more deadly carbon monoxide. Carbon monoxide puts people into coma and causes blisters by reducing circulation to the skin. Now Dr Baxter wondered: could the same be true of carbon dioxide?

PETER BAXTER: When we talked to survivors it was clear that carbon dioxide mixed with air had acted as a kind of anaesthetic to send people to sleep and had put them into coma. Lying for many hours and not moving resulted in a sluggish circulation in the skin so that it's possible that the effects could be due to a lack of oxygen getting to the skin with the, this blistering ulceration reaction occurring as a result.

NARRATOR: Suddenly it became clear to Peter Baxter that the people in Cameroon didn't have burns. They had pressure sores and blisters. The mystery of Lake Nyos was finally over, but now the scientists had worked out what had unleashed the monster from the depths of the lake they had to make sure it could never happen again.

BILL EVANS: This lake really is a timebomb. The gas that comes into this lake every day increases the danger of an eruption, so unless steps are taken to remove that gas it's almost a certainty that the lake would erupt again.

NARRATOR: In the end they came up with a solution remarkable in its simplicity. It was just like putting a straw into a bottle of soda. After years of experimentation, in January 2001 a team of Cameroonian and French scientists installed a pipe in Lake Nyos. As the valve was opened the pressure of the gas forced a jet of water 50 metres into the air. The lake was exploding again, but this time the Cameroonian scientists were in control. But the killer lake has not yet been tamed.

GREGORY TANYILEKE (Institute for Mining and Geological Research): with just this one pipe alone the calculations show that it will take close to 15 years or more just to remove the quantity that is presently stocked at the bottom of the lake and that's why we've gone ahead and suggested very strongly that we need more pipes. We have to make sure we bring the gas levels to acceptable limits within the shortest time possible.

NARRATOR: At least now there is some hope that the villagers will be able to reclaim their homes. Now they understood lake overturn scientists scoured the world looking for other lakes that could turn into killers and after years of searching they pinpointed just one, but it was a lake that could cause a disaster on an unimaginable scale: Lake Kivu in Rwanda. One of the largest and deepest lakes in Africa, it is two thousand times bigger than Nyos and two million people live around its shores. For the people who live around Lake Kivu, the idea that their lake is a killer is absurd. Every day they bathe in the hot springs by the lake and enjoy the warm, bubbly water, but these are bubbles of carbon dioxide. These springs are the very thing that makes Kivu a killer.

DECKERS NGARUKIYE (Rwanda Brewery): People here don't consider it a danger. They are used to the lake, but they know there is gas in the lake. They never go deep. They know they can't possible go to the depth the gas layers are at, so they don't worry about these gases.

NARRATOR: And there's more than just carbon dioxide. Kivu is home to another deadly gas, one that's highly explosive: methane. Since the 1980s a local beer factory has been extracting the methane from the lake to run the brewery, a project that the Rwandan government is keen to develop on a bigger scale. Until recently the people of Rwanda saw it as a blessing.

DECKERS NGARUKIYE: The big advantage is that we will have gas and cheap electricity that we can use everywhere in the country.

NARRATOR: The worry is that if the lake overturned it would not only release deadly carbon dioxide but that the methane might also explode. But when the Rwandan government looked into this there seemed to be no real threat. The levels of carbon dioxide in the bottom of the lake were lower than at Nyos. Kivu seemed safe, but then came news that would change everything. Lake Kivu had exploded many times before. The evidence came from a scientific snapshot into the past. 30 years ago Robert Hecky travelled to Kivu to take samples of the sediments on the bottom of the lake. It gave him a record going back thousands of years. What he saw when he got back to his lab turned out to be very bizarre. Mysterious dark layers that he had never encountered anywhere before stood out from his sample.

PROF ROBERT HECKY (University of Waterloo): We're going back in time as you go along this core. Then we can see a very profound break in the nature of the sediment forming in Lake Kivu at that time, a very distinctive dark brown material that was present and we can see that it was not only present then, but it seems to recur at least five times. I've never really seen anything like that before and I haven't seen anything like it since.

NARRATOR: He took a closer look at one of those dark layers, a layer that dated back five thousand years and he found something very strange. All living creatures in the lake had disappeared while huge amounts of vegetation had been swept in from the land. When he looked at each layer in turn the same process had happened almost every thousand years. Hecky was mystified, until the Nyos disaster.

ROBERT HECKY: Well I can remember suddenly a light bulb going on when I saw the reconstruction of Nyos and what they said had gone on, it suddenly fit.

NARRATOR: Just like at Nyos an explosion had killed all the fish and created a tidal wave which dragged in trees and plants from the shore. Robert Hecky realised he was looking at the forensic evidence of not one, but many examples of lake overturn at Lake Kivu.

ROBERT HECKY: We can say that perhaps every thousand years this lake has exploded. When is the exact trigger, what sets it off at that time we're not sure.

NARRATOR: No one knows for certain what triggered the last overturn at Kivu, but what that trigger could have been was clear for all to see in January 2002.

NEWSREADER (MICHAEL BUERK): An exploding volcano has half-buried an African city in molten lava. Rivers of flame have killed dozens of people. Hundreds of thousands are fleeing.

NARRATOR: Back in the US Bill Evans and George Kling were watching the pictures coming out of Rwanda.

GEORGE KLING: We were watching the reports of the lava flows come in and it was really difficult to find out how much lava there was and whether it was going to reach the lake. Lava is really scary because that's a huge amount of energy input to the lake and that amount of energy, much more than could be in a landslide, could actually trigger an overturn.

NARRATOR: They watched as the lava flowed into the lake moving down closer and closer to the layers containing the carbon dioxide. For three days the lava flowed into the lake and then it stopped. The lava hadn't got anywhere near the bottom layer holding the deadly gas. It seemed the danger had gone away, but it hadn't. The volcano was just a warning. The whole area was becoming unstable. Lake Kivu sits on top of a giant rift valley which was being pulled apart. In the aftermath of the eruption a huge crack opened up inside the volcano. It formed a tunnel carrying huge amounts of boiling lava deeper and deeper underneath the lake.

GEORGE KLING: The danger is not going to come from above, it's going to come from below, beneath the lake, lava pouring into the bottom and we know that that's happened in the past, we know that there have been great releases of gas in the past and so energy input from the bottom is what we have to watch for.

NARRATOR: As the rift is pulled apart the crack moves closer and closer to the bottom of the lake. The fear is that the heat of the lava would release the CO2 sending a cloud of suffocating gas to the surface, but this time there would be a terrible sting in the tail. The heat from the lava would ignite the methane gas.

GEORGE KLING: There's a double whammy really associated with that gas. The methane would come out of solution first and the methane at high enough concentrations is explosive, so there would be large explosions above the lake. That would be followed by the CO2 gas which would move out off of the lake where it would suffocate people living around the shores.

BILL EVANS: Well this would be a tragedy that would just dwarf the Nyos event. Nyos - about 1,800 people were killed. In the Lake Kivu basin possibly two million people could be killed by an overturn of Lake Kivu.

NARRATOR: The consequences are so terrifying it is now crucial that something is done. The safest way is to remove the gas.

BILL EVANS: Piping the gas out of Kivu is going to be a real challenge. It's a much bigger lake than Nyos and much deeper, so it'll take some research to figure out exactly how to get this piping system going, but it can be done. With enough pipes and the right kind of setup the gas can be removed from Lake Kivu.

NARRATOR: Difficult as it is to degas a lake as big as Kivu, there is no other option.

GEORGE KLING: We can't predict when a volcanic eruption would occur and trigger an overturn and even if we could there's no way that we could stop it. We can't keep the rift from pulling apart.

NARRATOR: With the rift valley still unstable it's a race against time to degas Kivu, but that will take millions of dollars of international aid. Only then will the people of Rwanda be safe.

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