A lake in Cameroon released a cloud of gas that killed 1,700 people in one night

On a single night in northwestern Cameroon, a quiet volcanic lake released an invisible cloud that rolled down surrounding valleys and suffocated nearly everyone in its path before dawn. Almost 1,750 people died in villages that had no warning and no way to see the danger coming.

What happened at Lake Nyos on August 21, 1986

The event is known as a limnic eruption, a rare phenomenon in which carbon dioxide dissolved deep in a lake’s water is suddenly released. At Lake Nyos, roughly 100,000 to 300,000 tons of carbon dioxide burst from the lake’s depths, sending the gas cloud rising into the air at nearly 100 kilometers per hour before it fell back toward the ground, since carbon dioxide is heavier than the surrounding air. As it spread, the cloud hugged the terrain and flowed downhill through the valleys surrounding the lake, moving at 20 to 50 kilometers per hour and remaining thick enough to suffocate people for a stretch of roughly 23 kilometers.

A death toll concentrated in four villages

The gas struck the villages of Nyos, Kam, Cha, and Subum while most residents were asleep, ultimately killing 1,746 people and roughly 3,500 livestock, according to the documented history of the Lake Nyos disaster. Survivors described losing consciousness almost instantly and waking hours later to find family members and neighbors dead around them, with pathology studies later confirming that carbon dioxide asphyxiation, not any secondary toxin, was the direct cause of death. Roughly 4,000 people who fled the area afterward reported respiratory damage, skin lesions, and paralysis from their exposure to the gas.

What triggered the release remains unresolved

Scientists still have not settled on a single confirmed trigger for the eruption. The leading hypotheses include an underwater landslide disturbing the lake’s layered water column, a small volcanic disturbance on the lake bed, or a period of unusually cool rainfall on one side of the lake shifting the balance between its layers. Some researchers argue no external trigger was even necessary, since the chemistry of carbon dioxide dissolved in cold water is inherently unstable once concentrations reach a critical threshold, meaning the lake could, in principle, destabilize on its own once enough gas had accumulated at depth.

The mechanics of a lake that can explode

Lake Nyos sits in a volcanic crater fed by magma below, which slowly leaches carbon dioxide into the deepest layers of the lake over time. Normally, a stable layer of cold, gas-saturated water stays trapped beneath a warmer, less dense layer near the surface, preventing the dissolved gas from escaping. Once something disturbs that layering, whether a landslide or an internal chemical instability, the deep water rises quickly enough that the drop in pressure allows the dissolved carbon dioxide to fizz out of solution all at once, similar to shaking and then opening a bottle of carbonated soda. Researchers estimate that around 1.2 cubic kilometers of gas escaped in the 1986 event, along with enough iron-rich deep water rising to the surface that the lake’s normally blue color turned red for a period afterward.

Engineering a lake so it cannot do this again

The scale of the disaster pushed scientists to design a way to prevent a repeat, and the solution settled on was a controlled degassing system: long pipes lowered into the lake that lift gas-saturated water from the depths, allowing the dissolved carbon dioxide to escape gradually and safely rather than building up to dangerous concentrations. The first permanent degassing pipe was installed at Lake Nyos in 2001, and two additional pipes followed in 2011. By 2019, monitoring data indicated the degassing process had reached a steady state, with even a single one of the installed pipes capable of keeping the lake’s carbon dioxide levels at a safe threshold indefinitely without external power.

A warning that reshaped how scientists study other lakes

After the Lake Nyos disaster, researchers began surveying other volcanic lakes in Africa’s Rift Valley region for similar gas buildup, most notably Lake Kivu on the border of the Democratic Republic of the Congo and Rwanda, a lake roughly 2,000 times larger than Nyos. Early studies found evidence that Lake Kivu had also become supersaturated with dissolved gas and that similar outgassing events may have occurred there roughly every thousand years, though a later reanalysis published in 2020 found methodological problems in that earlier work and concluded the risk of a gas eruption at Kivu did not appear to be increasing over time. The Lake Nyos disaster remains, alongside a smaller 1984 event at Cameroon’s Lake Monoun, one of only two confirmed limnic eruptions recorded in modern history, and it continues to serve as the primary case study for understanding how a body of water can turn into a hazard capable of killing without warning.

This article was created with the assistance of AI and reviewed by an editor.

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