Lake Nyos once belched a cloud of carbon dioxide that suffocated a whole valley

On the evening of August 21, 1986, villagers around a quiet crater lake in northwestern Cameroon went to sleep as usual. By morning, nearly 1,750 of them were dead, along with roughly 3,500 head of livestock, and the surrounding valleys looked, in the words of reporters who arrived soon after, like the aftermath of a neutron bomb. No explosion had occurred. Instead, Lake Nyos had released an enormous invisible cloud of carbon dioxide that rolled down the surrounding hillsides and suffocated everything in its path.

A Lake Built on Top of a Hidden Reservoir of Gas

Lake Nyos sits inside a volcanic crater in Cameroon’s Northwest Region, formed by a maar explosion that geologists believe occurred within the last 12,000 years. A pocket of magma lies roughly 80 kilometers beneath the lake, and it continuously leaks carbon dioxide upward through natural springs that feed into the lake’s deep water, where the gas dissolves and accumulates under pressure. Lake Nyos is one of only three lakes on Earth known to be saturated with carbon dioxide in this way, alongside nearby Lake Monoun and Lake Kivu on the border of Rwanda and the Democratic Republic of the Congo. Over years and decades, the deep water becomes increasingly supersaturated, setting the stage for what scientists call a limnic eruption.

The Night the Lake Turned Over

What triggered the 1986 eruption has never been established with certainty. Most geologists suspect a landslide disturbed the lake’s stratified layers, while others have proposed a small volcanic event on the lakebed or a sudden influx of cool rainwater. Some researchers argue no external trigger was even necessary, since carbon dioxide solubility decreases as water warms, making the lake’s layered structure inherently unstable over time. Whatever set it off, the result was a rapid mixing of the gas-saturated deep water with the layers above it, allowing the dissolved carbon dioxide to fizz out of solution in a rapidly escalating chain reaction, much like the sudden release of pressure from a shaken can.

A Cloud That Raced Downhill Faster Than a Car on the Highway

Scientists estimate the eruption released between 100,000 and 300,000 tons of carbon dioxide, roughly 1.2 cubic kilometers of gas in total. The cloud initially shot upward at nearly 100 kilometers per hour, and because carbon dioxide is about 50 percent denser than ordinary air, it quickly fell back to the surface and hugged the ground as it spread. Moving through the valleys at 20 to 50 kilometers per hour in a layer roughly 50 meters thick, the gas stayed concentrated enough to be lethal for approximately 23 kilometers, engulfing the villages of Nyos, Kam, Cha, and Subum along the way. The eruption also sent a wall of water and foam at least 25 meters high sweeping across one shore of the lake, and the water’s normally blue color turned a deep red as iron-rich water from the depths rose and oxidized on contact with air.

Survivors Described Waking Into a Silent Nightmare

Because carbon dioxide is odorless and colorless at lethal concentrations, many victims lost consciousness in their sleep without any warning. One survivor from the village of Subum later described regaining consciousness unable to speak or open his mouth, finding his daughter and neighbors already dead, and riding a motorcycle through a Nyos that showed no sign of any living thing. Medical researchers who studied the aftermath, published in a 1989 analysis in The BMJ, concluded that most deaths resulted directly from carbon dioxide asphyxiation, while survivors near the cloud’s edge suffered skin lesions from prolonged pressure against the ground as they lost consciousness, according to the account preserved on Wikipedia’s entry on Lake Nyos.

Engineers Eventually Built a Fix Directly Into the Lake

The scale of the tragedy pushed scientists to design a way to prevent a repeat event. Beginning in the mid-1990s, researchers led by French scientist Michel Halbwachs tested a method of slowly venting the trapped gas: a pipe lowered into the lake’s depths lifts saturated water upward until the drop in pressure causes the dissolved carbon dioxide to bubble out on its own, creating a self-sustaining fountain that continuously bleeds off gas without needing external power once started. The first permanent degassing pipe was installed at Lake Nyos in 2001, with two additional pipes added in 2011. A 2019 assessment found that the degassing process had reached a steady state, meaning even a single one of the installed pipes could keep carbon dioxide levels at a safe threshold indefinitely.

A Second, Quieter Danger Still Lingers

Even with the gas threat under control, Lake Nyos poses a separate risk that has drawn scrutiny from the United Nations. The natural volcanic rock dam holding back the lake’s water has been weakening for years due to erosion, and a 2005 inspection by a UN team confirmed the wall had thinned considerably at its narrowest point. Engineers have discussed reinforcing the dam or cutting a controlled channel to lower the water level and ease pressure on the barrier, since a sudden collapse could send a large volume of water rushing downstream into Cameroonian and Nigerian communities. Despite the lingering risks from both gas and water, families have continued to resettle the area, citing the unusual fertility of the volcanic soil and a desire to return to ancestral land.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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