On This Day: August 21, 1986 — A Lake in Cameroon Exhaled and 1,746 People Died in Their Sleep

At roughly 9:30 PM on August 21, 1986, a low rumbling rolled across the hills above Lake Nyos in Cameroon’s Northwest Province. It lasted about 20 seconds. Then a smell like rotten eggs drifted through the darkness. By morning, four villages were silent. Not destroyed. Not flooded. Silent. Buildings stood untouched. Fields were green. The sun came up as it always did. But 1,746 human beings and 3,500 cattle were dead, scattered where they had fallen the night before, as if the world had simply stopped between one breath and the next.

The Lake Nobody Suspected

Lake Nyos sits at 1,091 meters above sea level in the Oku volcanic field of northwest Cameroon — a brilliant blue crater lake roughly two kilometers wide and 200 meters deep. For generations, the local Kom and Bafut peoples had farmed its fertile shores. The lake was beautiful and reliable. Cattle drank from it. Children swam in it. There was no local legend warning of sudden death. The danger wasn’t written in the landscape.

Beneath the surface, the story was very different. Lake Nyos sits over a magmatic region where volcanic gases — primarily carbon dioxide — seep upward through fissures in the lake bed. Under normal circumstances, gases rise through water and escape into the atmosphere. But Lake Nyos is what limnologists call a meromictic lake: its water layers don’t mix. Cold, dense, CO2-saturated water sits at the bottom while warmer water caps it from above. The boundary between those layers — the thermocline — acts like a seal. Every year, more volcanic CO2 seeps in. Every year, that seal holds a little more pressure.

Scientists later calculated that by 1986, the bottom 50 meters of Lake Nyos held somewhere between 250 and 300 million cubic meters of dissolved carbon dioxide — roughly equivalent in mass to what a medium-sized coal power plant emits over two years, all compressed into the cold black water beneath a peaceful-looking crater lake. Something just had to disturb it.

What Happened at 9:30 PM

Geologists still debate the precise trigger. The most widely accepted explanation is a landslide along the lake’s western shoreline that suddenly destabilized the water column. When a significant volume of material entered the lake, it disturbed the stratified layers, causing an abrupt drop in pressure on the CO2-saturated bottom water. The result was the same as releasing the cap on an overpressured bottle of soda — except on a scale measured in hundreds of millions of cubic meters.

The eruption sent a column of water, foam, and CO2 surging roughly 80 meters into the air. The lake’s famously clear blue water turned a deep brick red as iron-rich bottom water oxidized on contact with the surface. The water level dropped by about a meter. A small tsunami — estimated at 25 meters in height — crashed over the northern shoreline and stripped vegetation from the hillside to an elevation of 80 meters above the water line. Trees were toppled. Soil was peeled away. The physical force of the eruption was extraordinary. What came next was invisible.

Carbon dioxide is roughly 1.5 times heavier than air. It doesn’t rise — it flows, like a liquid, filling valleys and following terrain downhill. The gas cloud from Lake Nyos, estimated at 50 to 100 meters in depth, moved north at speeds up to 72 kilometers per hour. At concentrations above 10%, carbon dioxide causes rapid loss of consciousness. At 20% and above, death follows within minutes. The cloud carried far higher concentrations than that as it poured down the slopes toward the sleeping villages below.

The Villages That Didn’t Wake Up

The gas reached the village of Lower Nyos first. Lower Nyos had roughly 800 inhabitants. A single woman and child survived. Everyone else was dead before they could regain consciousness. The gas rolled on to Cha, Subum, Fang, and Mashi — smaller settlements along the valley floor that the CO2 cloud reached within minutes of leaving the lake. In Subum, a witness later described the birds and insects going silent before anything else, a sudden eerie stillness before the smell arrived.

Rescue teams who reached the area about 36 hours later — delayed partly by difficulty reaching a remote region and partly by fear of further eruption — described something that defied easy description. Buildings were intact. Pots sat on fires that had burned out overnight. Food was on tables. Clothes hung from lines. Cattle and dogs lay in the road. People were found in their beds, in their doorways, in the middle of paths, exactly where the gas had found them. There was no evidence of flight, no sign of panic, no struggle — just absence, everywhere, the absence of 1,746 people who had been breathing one moment and were gone the next.

Those who survived — perhaps 4,000 in the wider area, most on higher ground — woke to comas lasting anywhere from six to 36 hours. Joseph Nkwain, a survivor from Subum, later described to researchers what it felt like: “I could not speak. I became unconscious. I could not open my mouth because then I smelled something terrible.” Several survivors, upon regaining consciousness and finding their families dead around them, took their own lives. The psychological aftermath of Lake Nyos was as long and as deep as the lake itself.

The Science No One Had Seen Before

Geologists had a word for what happened — a limnic eruption — but no one had documented one at this scale in the modern era. The first indication that such a thing was even possible had come just two years earlier, and been largely ignored.

On August 15, 1984, 37 people died near Lake Monoun, another crater lake in the same volcanic field roughly 100 kilometers to the south. Scientists investigating that event concluded that a sudden release of CO2 was responsible — and submitted their findings to the journal Science. The paper was rejected as implausible. When Lake Nyos killed 1,746 people in 1986, the scientists who had written that paper were vindicated in the worst possible way. The editors of Science published a fast-tracked follow-up. The world now had a word, and a definition, for what a killer lake could do.

The physics are straightforward, though the scale remains hard to absorb. CO2 dissolves in cold water under pressure, just as carbonation is held in a sealed bottle. As long as a lake’s thermal stratification holds, the gas stays dissolved at depth. A landslide, a rockfall, a sudden temperature shift — anything that mixes the water layers — can trigger a runaway degassing event. Dissolved gas becomes free gas, free gas drives more water upward, that water degasses as pressure drops, and the whole process accelerates. Once started, it cannot be stopped from the outside. It runs until the pressure is exhausted.

Survivors Who Woke Up to the End of the World

Halima Suley was a cowherd who lost four of her children, 31 other family members, and 400 head of cattle in a single night. She had been at a slightly higher elevation when the gas passed through. “I’m no more thinking about the disaster,” she told a Smithsonian writer years later. “I’m thinking about the children I have now.” She rebuilt on an elevated compound, above the valley floor that had funneled the gas into the villages. She had five more children after 1986. She never moved away from the lake.

For hundreds of displaced survivors, the aftermath was prolonged and politically complicated. The Cameroonian government barred residents from returning to Nyos and the surrounding villages for years, citing safety concerns. Families who had farmed the rich volcanic soil for generations were housed in refugee camps far from their land. The lake, now rust-red rather than blue, sat empty above empty fields.

Mercy Bih was 12 years old when the eruption happened. She lost all 26 members of her extended family. By the time researchers interviewed her years later, she had become the proprietor of the Lake Nyos Survival Good Faith Club restaurant in the nearby town of Wum — named for the survivors of that August night. She was keeping the memory alive in the only way available to her: by feeding people and staying open.

How Scientists Raced to Defuse a Ticking Lake

The eruption of 1986 had not solved Lake Nyos’s fundamental problem — it had only momentarily relieved the pressure. Volcanic seeps continued loading the lake bottom with CO2. By the early 1990s, monitoring data showed the lake recharging. Scientists who understood what they were looking at had a clear picture of the danger: without intervention, Lake Nyos would eventually erupt again. And the population in the surrounding valleys — already traumatized by 1986 — would be at the same risk.

The solution was engineered, not geological. Michel Halbwachs, a French engineer at the University of Savoy, designed a degassing system using a basic physical principle: if you pipe CO2-saturated bottom water to the surface in a contained tube, the pressure drop as it rises causes it to degas in a controlled, manageable way rather than catastrophically. The gas escapes into the atmosphere at low concentrations, harmlessly. It’s the same principle as cracking a carbonated bottle slowly rather than shaking and uncapping it.

On January 30, 2001 — fifteen years after the disaster — the first degassing pipe at Lake Nyos was activated by a joint Cameroon-France-Japan-USA team. A 50-meter fountain of water and carbon dioxide erupted from the mouth of the pipe. Those watching described it as startling and almost beautiful. By 2011, two additional pipes had been installed. By 2019, the system had stabilized the lake’s CO2 levels enough to sustain itself without external power. Lake Nyos now has a system that, barring catastrophic new volcanic activity, should prevent another eruption on the scale of August 21, 1986.

Fifteen years after that, the lake is blue again on most days. Scientists continue to monitor it. The villages of Lower Nyos and Cha have never been fully repopulated. Some survivors returned to farm the surrounding land. The gas seeps continue, below the surface, as they have for millennia. The degassing pipes run day and night, draining away the invisible killer before it can gather enough pressure to move again.

Lake Nyos is one of only three lakes in the world confirmed capable of a limnic eruption — the others are Lake Monoun and Lake Kivu, the latter of which sits above approximately two million people. On August 21, 1986, the lake that most of the world had never heard of killed 1,746 people between one breath and the next, with a gas they could not see, in a geological event that had no modern precedent. If this story stopped you cold, read about another August night in 1987 when a single child survived a plane crash alone, or how the failed Soviet coup of August 1991 brought down an empire in three days. August, it turns out, has a long history of nights that changed everything.

Sources

1. Smithsonian Magazine: “Defusing Africa’s Killer Lakes” — Comprehensive account of the disaster and degassing efforts, with direct survivor interviews
2. Wikipedia: Lake Nyos Disaster — Scientific overview, casualties, and timeline with citations
3. HISTORY.com: Gas Cloud Kills Cameroon Villagers, August 21, 1986 — Date-confirmed historical account
4. Eos.org: Cameroon’s Lake Nyos Gas Burst: 30 Years Later — Scientific review of the event and subsequent research
5. USGS: Degassing Lakes Nyos and Monoun — Defusing Certain Disaster — Official US Geological Survey publication on the degassing solution