On the morning of August 22,  1986, a local doctor was riding his bike towards the village  of Nyos in northern Cameroon, when he spotted a dead antelope  along the side of the road.

As he pressed on, he found two  dead rats, then a dead dog, then a whole host of other dead animals. At first, he thought they might have been killed in a freak lightning strike.

But as he arrived in Nyos, he soon realised that whatever had killed these animals had extended its reach much farther than lightning could. Among the huts, there were  dead bodies strewn everywhere. Human bodies.

Not a single person was left alive. But there was no sign of what had killed everyone. The doctor ran for his life.

Over the next few days, it became clear that the people of Nyos and several other nearby villages had asphyxiated from  breathing in carbon dioxide. But figuring out what caused  this tragedy took much longer. And it would be more than three decades before villagers could be sure that  the area was safe again.

This is the story of the Lake Nyos  Disaster, and the surprisingly low-tech method of preventing a  similar tragedy from happening again. [♪INTRO] Located in Central Africa,  Cameroon is a country with a bit of everything: desert, savannah,  mountains, and tropical rainforest. As a result, it’s often described  as ‘Africa in miniature’. And among that varied landscape,  it also has volcanoes.

The so-called Cameroon line  is a nearly straight line of volcanic rocks that scientists  think formed when the entire continent of Africa rotated  about 80 million years ago. For context, that’s like 30 million years after Africa and South America had their major breakup. All that rotation failed to rip Africa  itself apart, try as it might, but it did cause enough magma to spill forth  and make a bunch of volcanoes.

In the ocean, volcanoes along the Cameroon line formed several islands,  including Sao Tome and Principe. And on land, we got the Oku  Volcanic field, which extends for around 100 kilometers along  Cameroon’s northwestern border. Within this region, there  are a bunch of big volcanoes, like Mount Oku, Mount Babanki and Mount Nyos.

And pretty much the whole landscape has been shaped by volcanic activity. In particular, there are more than  a thousand cinder cone volcanoes, and around 50 volcanic lakes called maars, which have been blasted out of the  bedrock by explosive eruptions. One of those maars is Lake  Nyos, which is more than 200 meters deep and covers an area  of around 1.5 square kilometers.

Scientists think a volcanic eruption  excavated the lake’s crater in the granite bedrock about 12,000  years ago, and the water inside it is held back by a rim of material  left behind by pyroclastic flows. But while things might have  seemed quiet at the surface over the last few millennia,  at the bottom of Lake Nyos, a disaster had been brewing. See, there’s a network of hydrothermal cracks and vents which underlies the  entire Oku volcanic field.

And it turns out that one of those vents releases into the waters of Lake Nyos. Quietly, these so-called soda springs had been leaking carbon dioxide gas out of the lakebed. Now normally, that wouldn’t  be too much of a problem, since the gas can dissolve in water.

And the pressure of 200  metres of lake on top of it is enough to keep it in place at those low depths. But that all changed on the  night of August 21st, 1986. Around pm, a giant cloud  of carbon dioxide was suddenly expelled from the lake at up  to 100 kilometers per hour.

It filled the lake basin, extending  100 meters above the crater rim. But then, because carbon dioxide  is heavier than air, that cloud spilled over the northern rim and  surged down across the landscape. Travelling at up to 50  kilometers per hour, and with a thickness of about 50 meters, the  cloud of gas filled the valleys, flowing through the nearby  villages of Nyos, Chah, and Subum.

And everywhere this concentrated  cloud touched, it brought death. Normally, CO2 makes up just a tiny  fraction of the air we breathe: around 400 molecules in every million. But when that concentration rises, the human body can quickly become overwhelmed.

We’re constantly making carbon dioxide  in our cells, but our circulation and respiratory systems work hard  to get it out as soon as possible. In fact, when we hold our  breath, the signals we get from our brain are responding to  an increase in the amount of CO2 in the blood, rather than a lack of oxygen. When the carbon dioxide  levels outside the body rise, you start experiencing some of the same symptoms.

Like a desperate need to breathe called air hunger a fast heart rate, and fast breathing. However, at high enough concentrations, the carbon dioxide levels in the air match the levels coming out of your body. There’s no concentration gradient for the gas to diffuse out of your blood.

Meaning you can’t get rid of it. According to the CDC, carbon  dioxide is immediately dangerous to life and health when  it hits 50,000 parts per million, or around 5% of the air you’re breathing. That’s more than 100 times  its natural concentration.

At this point, CO2 levels  inside the body keep rising, from both internal and external sources. Any gas dissolved in your body  fluids increases their acidity, which has a profound impact on  how critical body functions work. Enzymes and other proteins lose  their shape, so they stop working.

Blood cells can’t carry oxygen as effectively. You start to feel fuzzy and light headed as your cellular machinery shuts down. Eventually, the neurons in your  central nervous system stop firing, sending your body into unconsciousness,  coma, and potentially death.

This can happen in mere minutes when the concentration hits 10% of the air. And of course, if carbon dioxide is  the main gas you’re breathing in, then you’re not going to be getting the oxygen your cells need to keep functioning. At Lake Nyos, experts estimated  that around 1.2 cubic kilometers of carbon dioxide was released  from the lake into a single cloud.

That cloud would have started out at 100% CO2, and because of how fast it moved, it wouldn’t have had much chance to mix with the surrounding air until it was very spread out. So the inhabitants of the surrounding villages would have had little warning,  and no chance to avoid the invisible cloud of death  that descended from the lake. In total, around 1,800 people were killed, along with roughly 8,000 livestock animals and uncounted numbers of wildlife.

It took over a day for a rescue group to arrive, and nobody is sure of the precise death toll. People had already started  burying the bodies in mass graves, and many survivors had simply  fled the area, not knowing what had caused the tragedy that had  killed their friends and family. It was clear that all those people had been killed by breathing in carbon dioxide, and experts were pretty sure it had something  to do with the volcanic lake at the center of the disaster zone.

But they couldn’t agree on exactly how. Seven months after that tragedy, in March 1987, Cameroon’s president hosted  an international science conference for 100 experts  from a range of disciplines, to unravel the geological  mechanism of CO2 release. And we’ll get to the results of that unraveling, right after this ad break: This SciShow video is supported by DeleteMe, the hands-free subscription service that removes your personal information from hundreds of data brokers and people search sites.

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Get 20% off DeleteMe consumer plans when you go to joindeleteme.com/SCISHOW and use promo code SCISHOW at checkout, or use the QR code on-screen. At the end of the 5-day meeting,  the scientists had basically settled into two camps: one which thought  the carbon dioxide had come from a volcanic eruption beneath the  lake, and another which blamed the gradual buildup and release of  dissolved gas in the lake itself. The eruption hypothesis fits with what we know about how volcanoes work.

Without warning, hot magma  underground can superheat groundwater, causing it to flash into  steam and explode upwards, carrying volcanic minerals and  gases into the air without any lava. This is what’s known as a phreatic eruption. Some survivors reported a sulphurous  smell in the air at the time of the disaster, while others heard  rumbling and saw a surge of water, both of which seem to fit  with a volcanic eruption.

But on the other hand,  there was no ground movement noticed by survivors, or recorded by the nearest seismometer 200 kilometers away. Plus, later geochemical measurements found there were none of the other volcanic minerals you’d expect to see left behind in the lake water. So instead, the other camp  of experts was convinced the carbon dioxide had been  gradually seeping into the lake and saturating the bottom layers with the gas, until some external force triggered its release.

At the lake’s maximum depth of around 200 meters, water can hold up to 15 times  its own volume of carbon dioxide. And it’s been estimated that every  liter of water in the lower parts of Lake Nyos contained between  one and five liters of CO2. The weight of water above this  saturated layer would have acted like a cap on a soda bottle, keeping  all of that carbon dioxide inside.

But if the status quo is ever disturbed, if something displaces the  gas-rich layer at the lake bottom, that layer rises up and suddenly  experiences less pressure. Like opening that soda bottle,  the dissolved carbon dioxide starts to come out of solution and forms bubbles. Those bubbles rise turbulently,  pulling in more gas-rich water and forming ever more  bubbles, and so on and so on.

So the final result is a huge outburst  of carbon dioxide at the surface. All that fits with the  observations from Nyos as well: the rumbling and surging of water could have come from gas bubbling violently  out of the lake surface. Plus, the lake level had dropped by about a meter after the event, which could be explained by the loss of all that gas  from the lake’s bottom waters.

Today, most scientists have  come to agree that this is the most likely explanation  for the Lake Nyos disaster. And it’s not the only one. Two years earlier, 37 people died  from carbon dioxide expelled from another maar lake, Lake Monoun,  less than 100 kilometers away.

The term limnic eruption, literally  “an eruption from a lake”, was coined to describe these two  carbon dioxide emission events. They are, however, the only limnic eruptions in history that scientists are aware of. What still remains unknown is what triggered the  release of gas from Lake Nyos’s  bottom in the first place.

It may have been a landslide,  since a relatively recent scar was spotted at the edge of the  lake during aerial surveys, but nobody knows exactly when it happened. Alternatively, it may have been an influx of water from a ton of recent rainfall. The evening of the disaster  was reportedly a rainy one, and the cold, fresh water could have sunk to the bottom and disturbed the lake’s lower layers.

Or there could have been  no external trigger at all! The bottom layers could have  just become oversaturated, and any further carbon dioxide was forced to form bubbles despite the pressure. Without any certainty about causes or triggers, the surviving population around Lake Nyos were understandably cautious.

Fifteen thousand survivors fled  the area, and over 4,000 villagers were resettled to temporary camps in  the first week after the disaster. Eventually, they were relocated to permanent sites far from the potentially deadly lake. But these villagers had  lost their ancestral homes, along with prime agricultural and pastoral land that they depended on for  their lives and livelihoods.

So at that 1987 conference, a  long term solution was sought to ensure a disaster like  this would never happen again. The solution turned out to be a bunch of flexible plastic pipes and a controlled recreation of the process that caused the  eruption in the first place. It was nicknamed the Lake  Nyos Organ Pipes project, and tested throughout the 90s.  Permanent installations started in 2001.

Essentially, the project featured  long plastic pipes installed vertically into the waters of  both Lake Nyos and Lake Monoun. Using a small pump, operators could trigger the creation of bubbles at the bottom of the pipe. These bubbles would expand and  rise, pushing the gas-water mixture up the pipe and pulling more carbon  dioxide-rich water into the bottom.

This created a self-siphoning process that continued without any external input. Up at the surface, the gas  and water was expelled from the pipe with such force that it  created a continuously flowing jet, basically a fountain, in the middle of the lake. When this permanent siphoning began, powered by high gas concentrations, the jet reached a height of 46  meters above Lake Nyos’s surface.

But over time, the gas concentration  and therefore the jet height decreased until it was just a  spray less than a meter high. So we had a clear visual indicator that at least some progress was being made. And measurements of the gas  concentration at the bottom of the lake confirm the success  of the degassing project.

In 2001, water at the bottom  of lake Nyos started out containing 10 liters of carbon  dioxide for every liter of water. That’s worryingly close to the saturation point. But by 2018, this had been reduced to just 1.35 liters of CO2 per liter of water.

Lake Nyos and Lake Monoun won’t  ever be carbon dioxide-free. The volcanic gas leaking in from the Oku Volcanic Field will make sure of that. But now, the hazardous levels have been reduced, and the carbon dioxide levels can’t  rise higher because any excess will immediately be degassed  through the permanent organ pipes.

With just a few plastic pipes in each  lake, and a clever bit of physics, the threat of another limnic  eruption has been eliminated. Most natural disasters are  unexpected and uncontrollable. But just this once, it seems scientists and engineers have triumphed over the Earth. [♪OUTRO]