On the night of August 21, 1986, a quiet crater lake in the hills of northwestern Cameroon let out an enormous breath of carbon dioxide. The invisible gas poured down the valleys below and killed about 1,700 people, many of them in their sleep, along with some 3,500 livestock. There was no fire and no lava, and almost no warning. Lake Nyos had simply burped up the gas it had been storing for years. Here is how a lake can do that, why it almost happened again, and how a few plastic pipes now keep it from happening a second time.
The night the lake went silent
Lake Nyos sits high on the side of an old volcano, about 315 kilometers (196 miles) northwest of Cameroon’s capital, Yaoundé. It fills a maar, a round crater blasted out when rising magma hit groundwater. The crater is about 1.8 kilometers (a little over a mile) across and about 208 meters (682 feet) deep. Farming and herding villages lay in the valleys around it: Nyos, Cha, Subum, and others.
At about 9:30 that Thursday evening, people near the lake heard rumbling that lasted perhaps 15 to 20 seconds. One man walked to a lookout and saw a white cloud rise from the water and a surge of water slosh across the lake. That is the account a U.S.-led scientific team, arriving six days later, pieced together from survivors and published in the journal Science in 1987.
Many people reported a smell like rotten eggs or gunpowder and a feeling of warmth, and then they passed out. Those who survived woke between 6 and 36 hours later, weak and confused. Oil lamps had gone out even though they still had oil in them. Family members and animals lay dead around them. Birds and insects vanished from the area for at least two days. Plants were mostly untouched.
Reporters who reached the area said it looked like the aftermath of a neutron bomb, a weapon meant to kill people while leaving buildings standing. At first, many doctors thought the victims had been poisoned by sulfur gases, the ones that smell of rotten eggs. Later work showed that carbon dioxide, which has no smell at all, did the killing. Breathing high levels of carbon dioxide can cause sensory hallucinations, including phantom odors, which may explain at least some of the smells people reported.
Carbon dioxide, the quiet killer
Carbon dioxide (CO2) is the same gas we breathe out and the one that puts the fizz in soda. It makes up only about 0.04 percent of normal air. In small amounts it is harmless. In large amounts it pushes oxygen out of the way.
The U.S. Geological Survey (USGS) warns that air with more than about 3 percent carbon dioxide can quickly cause headaches, dizziness, a racing heart, and trouble breathing. Above about 15 percent, it causes unconsciousness and death within minutes. The cruel part is that a person gasping for air breathes in even more of it.
Carbon dioxide is also about 50 percent heavier than air. So instead of drifting away, the cloud from Lake Nyos hugged the ground and flowed downhill like water, filling the valleys. Estimates of how much gas came out range from about 100,000 to 300,000 tons, and some go higher. The cloud was roughly 50 meters (160 feet) thick and moved at 20 to 50 kilometers an hour (about 12 to 31 miles an hour). The first scientific team traced the deadly gas up to about 10 kilometers (6 miles) from the lake, and later accounts put deaths as far as 25 kilometers (16 miles) away.
The lake itself was changed, too. Its blue water turned a muddy red, because iron-rich water from the bottom had risen to the surface and rusted in the air. The lake level dropped by about a meter. Scientists later found signs that a wave at least 25 meters (82 feet) high had swept one shore.
How a lake stores a deadly gas
The gas did not come from the sky or from a sudden eruption. It had been building up in the lake for years. Deep under this region, magma gives off carbon dioxide, which seeps up through the rock and into springs at the bottom of the lake. Chemical tests, including helium isotopes typical of gas from magma, showed the gas came from a magma source, but with no sign of a direct volcanic eruption.
A soda bottle is the easiest way to picture what happened next. Water can hold a lot more carbon dioxide when it is under pressure and when it is cold. A sealed soda holds its fizz because the gas is pressed into the liquid. Open the cap, the pressure drops, and bubbles rush out. That rule is called Henry’s law.
The bottom of Lake Nyos is like a sealed soda. The water down there is cold and under the weight of some 200 meters of water above it, so it can hold huge amounts of gas. Most lakes would not keep that gas for long, because they turn over at least once a year, mixing top and bottom and letting the gas escape a little at a time. Lake Nyos is different. It is meromictic, which means its layers stay separate and do not mix. The heavy, gas-loaded water stays at the bottom, and more carbon dioxide piles up every year. Only about one lake in a thousand is layered this way.
Nobody knows exactly what popped the cap in 1986. Many geologists suspect a landslide into the lake. Others point to cold rainwater sinking on one side, a gust of wind, or a small eruption under the water. Some scientists argue that no trigger was needed at all, because a lake that full of gas is unstable on its own.
Whatever started it, the chain reaction is well understood. Once some deep water rises, the pressure on it drops and bubbles form. The bubbles make that water lighter, so it rises faster, releasing more gas, which pulls up more deep water behind it. Within minutes, a column of froth can turn the whole lake inside out. Scientists think a fountain of water and foam roughly 100 meters (330 feet) high shot up from Lake Nyos that night.
It had happened two years earlier
Lake Nyos was not the first. On August 15, 1984, Lake Monoun, another crater lake about 100 kilometers (62 miles) to the south-southeast, let out its own cloud of carbon dioxide. It killed 37 people. At the time, the deaths were a mystery, and some people suspected terrorism.
One detail from Monoun shows how the gas behaves. A truck carrying about a dozen people drove into the low ground near the lake, and its engine stalled, starved of oxygen. The people inside climbed out and died. Two people riding on top of the truck lived, because they were high enough to stay above the heavy gas. The USGS gives similar advice for volcanic areas today: carbon dioxide collects in low spots, and the border between breathable air and deadly gas can be so sharp that a single step uphill can save your life.
The Monoun disaster was investigated, but nobody expected a repeat. Two years later, Lake Nyos showed how much worse the same process could be. Together, the two are the only recorded limnic eruptions (gas eruptions from lakes) known to have killed people.
Refilling, year after year
After 1986, scientists kept measuring the lakes, and the numbers were alarming. Carbon dioxide was flowing back into Lake Nyos fast. By 2001, the gas pressure in its deep water had reached 15.6 bar, more than 15 times normal air pressure at sea level. Parts of both lakes were up to 97 percent saturated, meaning the water was very close to holding all the gas it possibly could. Early estimates suggested a gas burst like the one in 1986 could happen every 10 to 30 years if nothing was done.
There was a second worry, too. The rock wall that holds Lake Nyos in is a natural dam of volcanic rock, only about 45 meters (148 feet) wide at its narrowest point, and erosion has weakened it. A United Nations team that inspected it in 2005 warned that it could give way within 10 to 20 years. That has not happened, but if it did, as much as 50 million cubic meters of water could flood the valleys downstream, as far as Nigeria. Draining that much water would also lower the pressure on the deep layer, which could let more gas out.
Defusing the lake with a pipe
The fix that scientists came up with is simple and a little ironic. It uses the same runaway process that caused the disaster, on a small scale and under control.
A French team led by Michel Halbwachs, working with Cameroonian scientists, hung a long plastic pipe from a raft so that its bottom end sat in the deep, gas-heavy water. They pumped some water up the pipe to get it started. As the water rose, the pressure on it dropped, and the gas began to bubble out inside the pipe. The bubbles made the water in the pipe lighter, so it rose on its own, drawing more deep water in at the bottom. Once started, the pipe runs by itself with no power, shooting a fountain of water and gas into the air while the carbon dioxide drifts away in safe amounts.
The team tested the idea at Monoun in 1992 and at Nyos in 1995. In January 2001, the first permanent pipe went into Lake Nyos, with funding from the U.S. Office of Foreign Disaster Assistance. Its fountain reached tens of meters high. A pipe followed at Monoun in 2003, and two more there in 2006.
Some people feared that stirring the lake might set off another burst. A 2005 study in the Proceedings of the National Academy of Sciences (PNAS), led by University of Michigan biologist George Kling, found the opposite. The layers were holding steady, and the gas in each lake had dropped by about 12 to 14 percent. But the study also warned that one pipe at Nyos was far too slow, and that more pipes were needed.
Two more pipes went into Lake Nyos in 2011, with support from the United Nations Development Programme. They worked. Researchers tracking the lake reported that the amount of carbon dioxide dissolved in the lake fell from about 11.7 billion moles in 2006 to about 3.9 billion moles by March 2015, roughly a two-thirds drop. A 2020 paper by Halbwachs and colleagues concluded that the job was essentially done: a single pipe running nonstop can now remove carbon dioxide about as fast as nature adds it, keeping the lake at a safe level without any outside power.
People have gradually moved back into the area, drawn by family land and rich farm soil. A monitoring buoy was placed on the lake in 2014 to send back live readings of water temperature and conductivity, as part of an early-warning setup for the families now living nearby.
The much bigger lake next door to two million people
After Nyos, scientists went looking for other lakes that might do the same thing. They found one far bigger. Lake Kivu, on the border between Rwanda and the Democratic Republic of the Congo, covers about 2,700 square kilometers (1,040 square miles) and is up to 475 meters (1,558 feet) deep. About two million people live around its shores, including in the cities of Goma and Bukavu in Congo and Gisenyi in Rwanda.
Kivu’s deep water holds both carbon dioxide and methane, the main ingredient in natural gas. Estimates put it at roughly 256 cubic kilometers of carbon dioxide and 65 cubic kilometers of methane. Layers of mud on the lake bottom hint that something has killed off much of the life in the lake roughly every thousand years, possibly a gas release.
The good news is that Kivu is much further from the edge than Nyos was. A 2020 study in PLOS ONE, led by Fabian Bärenbold of the Swiss water research institute Eawag, compared several measuring methods side by side in 2018. It found that the deep water was only about half saturated at its most gas-rich depth, around 320 meters, and that gas levels did not appear to be rising after all. Earlier reports of a steady buildup seemed to come from measurement errors. When Mount Nyiragongo erupted in 2002, lava flowed into the lake, but it stopped well above the deep gas layer, and nothing happened.
Rwanda has also found a use for the gas. Since the 2010s, a project called KivuWatt has pumped deep water up to a barge, let the methane bubble out, and piped it ashore to run power plants. It was producing about 26 megawatts by 2016. Pulling the methane out removes some carbon dioxide too, though it is not clear that this alone will remove the danger.
Pockets of bad air elsewhere
Exploding lakes are rare because they need three things at once: a steady supply of carbon dioxide, a lake deep enough to hold it under pressure, and layers that never mix. Few places have all three. But smaller carbon dioxide traps show up in many volcanic areas.
Around Lake Kivu, people know them as mazuku, a Swahili word for evil winds: low spots where gas from the ground collects and kills animals that wander in. In 2006, three ski patrol members died at Mammoth Mountain in California after falling into a snow hollow around a volcanic vent that had filled with carbon dioxide. Gas seeping from the soil there has also damaged or killed plants in several spots on the mountain.
Some scientists think lakes may have done this long before anyone wrote it down. The Roman writer Plutarch described Lake Albano, near Rome, suddenly rising over its banks in 406 BC with no rain to explain it, and some researchers suspect a gas release. In Germany, the Messel pit, famous for beautifully preserved fossils some 47 million years old, is one place where a lake gas release has been proposed to explain why so many animals died at the water’s edge.
Lake Nyos today looks much like any other mountain lake, except for the fountain of water and gas rising from its degassing pipe. That fountain is the lake slowly letting out the breath it once released all at once.
Further reading and handy gear
Eco-autopsy of the Lake Nyos Disaster in Cameroon: 30 Years After Calamity — A short study by Gideon Aghaindum Ajeagah that pulls together the science, news coverage, and survivor accounts of the 1986 disaster and what came after.
Aranet4 Home Indoor CO2 Monitor — A battery-powered meter with an e-ink screen that shows carbon dioxide in parts per million, plus temperature and humidity, for checking whether a room needs fresh air.