Lake Nyos, in northwestern Cameroon, was the site of one of the quietest natural catastrophes of the 20th century.
On the night of Aug. 21, 1986, a sudden release of carbon dioxide killed 1,746 people and thousands of animals as they slept — no explosions, no fire, no warning of any kind.
It happened around nine in the evening. Survivors later described a smell like rotten eggs and a sound resembling distant thunder, lasting only seconds before everything went silent.
There were no signs of violence, no structural damage to homes. Only bodies — human and animal — were scattered across an area extending nearly 16 miles from the lake’s edge.
That detail baffled the first rescue teams to arrive. With no visible wounds and no evidence of an attack, early theories ranged from a curse to a chemical weapon.
The real explanation turned out to be geological.
The lake sits inside an ancient volcanic crater, and beneath its surface it had been quietly accumulating an extreme concentration of dissolved carbon dioxide for decades.
How Gas Builds Beneath a Deceptively Calm Lake
Nyos is what specialists call a meromictic lake: its water layers do not mix the way they do in most temperate lakes. Cold, dense water at the bottom stays separated from the surface water for years at a time.
This lack of mixing has a direct consequence: beneath the lake lies an inactive magma chamber that continuously releases carbon dioxide through fractures in the rock.
The gas dissolves into the deep water under pressure and remains trapped there, layer upon layer, for decades.
Before the disaster, the lake’s depths had accumulated between 100,000 and 300,000 metric tons of dissolved CO2 — equivalent, according to scientific estimates, to nearly one cubic mile of gas-saturated water.
What Triggered the 1986 Catastrophe
The exact cause of the destabilization remains a matter of debate.
The most widely cited theories point to a small landslide, a minor earthquake, or an influx of cold rainwater that disrupted the lake’s stratification. Any of these could have been enough to send the deep water rising toward the surface.
As it rose, pressure dropped, and CO2 began to escape — a process not unlike opening a shaken bottle of carbonated water.
The phenomenon is known as a limnic eruption: the sudden release of gases that have built up in deep water.
Within minutes, the gas surged upward at nearly 62 mph and formed a cloud that, being heavier than air, rolled down the crater’s slopes and into the populated valleys below. It displaced oxygen as it moved, asphyxiating everyone in its path — many of them asleep.
Two years earlier, in 1984, the nearby Lake Monoun had experienced a similar event, though on a smaller scale, killing 37 people.
At the time, the scientific community had not yet connected the two incidents to the same geological mechanism.
The Pipes That Prevent a Repeat Disaster
After confirming that CO2 was continuing to accumulate in the lake’s depths, French and German researchers independently proposed a degassing pipe system.
The first pipe was installed in 2001, and two more were added in 2011.
The principle is straightforward. A pipe is submerged to a depth of roughly 660 feet and draws up gas-saturated water. As the water rises, pressure drops and the CO2 begins to separate. This reduces the density of the mixture and keeps the flow self-sustaining — no external pump, no additional energy required.
According to the most recent report on the project, published in the Journal of African Earth Sciences, the system had reached a stable state by 2019. A single pipe, running continuously, is sufficient to offset the lake’s natural gas recharge and keep the risk under control indefinitely.
At Lake Monoun — now considered effectively degassed — the same approach served as a proving ground before being scaled up at Nyos.
The Risk That Persists in Other Lakes of the Region
Nyos and Monoun are not the only lakes on Earth capable of producing a limnic eruption.
According to data from the Smithsonian’s Global Volcanism Program, a third case exists — and it is considered far larger in scale. That lake is Kivu, on the border between Rwanda and the Democratic Republic of the Congo.
Kivu is roughly 1,500 times larger than Nyos and holds, in its deep layers, approximately 300 cubic miles of dissolved CO2 and 60 cubic miles of methane.
Unlike the Cameroonian lakes, it has never produced a catastrophic limnic eruption. Even so, its size and location — nearly two million people live within its basin — keep the international scientific community on alert.
Studies on Kivu are less conclusive than those on Nyos.
A 2020 investigation called into question parts of the risk calculations developed in 2005, suggesting that the probability of a similar event is lower than originally estimated.
Yet monitoring continues, because volcanic activity in the region — including that of the nearby Mount Nyiragongo — could alter the lake’s equilibrium at any moment.
Nearly four decades after the disaster, the floor of Lake Nyos is still releasing carbon dioxide, gram by gram, through a pipe that no one ever turns off.
