One August night, a calm lake, and by morning, nothing moved. On August 21, 1986, in the northwest of Cameroon, a limnic eruption at Lake Nyos caused the deaths of 1,746 people and nearly 3,000 head of livestock. No earthquake, no fire, not the slightest glow in the sky. Just an invisible gas, released from the water, that suffocated entire villages while they slept.
Key takeaways
- One August night: 1,746 people vanish without a sound or flame
- A CO2 cloud moving at 100 km/h creeping along the ground over 25 km around the lake
- Four decades on, no official memorial commemorates the victims
A cloud faster than a car on the highway
What happened that night has a precise term in geology: a limnic eruption. The eruption triggered the sudden release of roughly 100,000 to 300,000 tonnes of carbon dioxide. A mass of gas stored for years at the bottom of the lake, under pressure from the water, like the CO2 in a shaken bottle of soda. When the pressure gave way, it all burst forth at once.
The gas cloud first rose at nearly 100 km/h before descending, heavier than air, onto nearby villages, suffocating people and animals across 25 km around the lake. The speed of a high-speed train starting, the weight crushing everything in its path: CO2 crept along the ground, flooding the valleys like an invisible tide. The inhabitants had no warning signal. No suspicious odor, no unusual sound. Nothing that could have prompted them to flee in time.
The exact cause of this abrupt degassing remains debated. The trigger of this catastrophic degassing is not established. Most geologists suspect a landslide, but some believe in a minor volcanic eruption at the lake’s bottom. Two years earlier, a similar event had already struck Lake Monoun, in the same region, killing 37 people. Cameroon had just learned, at its own expense, a geological danger that science itself did not yet know how to name.
The “Nyos Organs,” or how to deflate a liquid bomb
The burning question after the tragedy remained: could the lake replay the same scenario? The scientists’ answer was unequivocal, yes, and action was needed. From 2001 onward, a French team led by Professor Michel Halbwachs installed a system as clever as it was artisanal. French engineers laid down degassing pipes to release the trapped gases in a controlled manner, a plastic tube descending to a depth of 203 meters. The principle relies on the siphon effect: once started, the water-CO2 mix rises on its own, erupts at the surface in a permanent geyser, and the gas disperses safely into the atmosphere. The device, locally nicknamed the “Nyos Organs,” was completed in 2011 by two additional pipes to accelerate the lake’s emptying.
The result, twenty years later, is rather encouraging. Michel Halbwachs explained that in 2001 there was a natural jet in the middle of Nyos Lake rising about 50 meters high. In the latest measurements taken in April 2016, it had dropped to two meters high. We are at about 90% of the gas that disappeared. There is no longer any risk of an explosion under current conditions. A real improvement, but not an eternal guarantee. A USGS-published study notes that the current CO2 content still represents 80% of the maximum level observed in January 2001, and that the depth of the lower chemocline could reach the height of the pipes within a few years, which would then gradually slow the rate of degassing. Emptying a lake of its gas takes decades, not seasons.
Forty years after, deaths without a burial
On the occasion of the catastrophe’s 40th anniversary, commemorated this year, a bitter reality resurfaces in Cameroon. On the night of August 21, 1986, a cloud of carbon dioxide escaped from Lake Nyos, suffocating 1,746 people in their sleep. The official toll, acknowledged by the Cameroonian authorities and the international scientific community, stands at 1,746 deaths and around 3,500 head of livestock decimated. Yet four decades later, displaced families are still waiting for genuine housing, and no official memorial pays tribute to the victims.
The gap left by the absence of explanations has also fed fanciful theories. Since 1986, conspiracy theories have circulated in Cameroonian public discourse, some voices claiming the catastrophe was provoked by military experiments conducted by Western powers in collaboration with the Cameroonian government. None of this is scientifically substantiated, but the mystery still surrounding certain lake mechanisms inevitably leaves room for speculation. In 2026, survivor associations organized their own ceremonies, due to a lack of national recognition.
There exists another lake, much larger, that currently worries scientists more: Lake Kivu, on the border between Rwanda and the Democratic Republic of the Congo. 2,000 times larger than Lake Nyos, it has also been deemed supersaturated, and geologists have found evidence that natural degassing occurs there roughly every thousand years. Yet millions of people now live on its shores, unlike the handful of villages around Nyos in 1986. The back-to-school season this week, with its share of people returning to the water’s edge, serves as a simple reminder: some lakes seem harmless, until the day they are not.
Sources: camer.be | fr.allafrica.com
