The Lake That Exploded
OnAugust21,1986,LakeNyosinCameroonsilentlyreleasedamassivecloudofCO2thatrolleddownsurroundingvalleys,suffocating1,746peopleand3,500livestockovernight.Theydidn'tevenwakeup.Itcanhappenagain—thelakeisstillthere.
A Lake on a Volcano
Lake Nyos sits at an elevation of 1,091 metres in the Oku Volcanic Plain of northwestern Cameroon. It is a maar lake — formed when a volcanic explosion created a deep crater that gradually filled with water. The lake is roughly 1.8 kilometres long, 1.2 kilometres wide, and 208 metres deep.
Beneath the lake, volcanic activity continues. Magma deep underground releases CO2, which percolates upward through fissures in the rock and dissolves into the cold, pressurised water at the bottom of the lake. The deep water of Lake Nyos is effectively a giant bottle of carbonated water — kept sealed by the pressure of the water column above it.
Under normal conditions, the CO2 stays dissolved. The deep water is cold, heavy, and doesn't mix with the warmer surface water. Tropical lakes like Nyos rarely experience the seasonal turnover that mixes deep and surface water in temperate lakes. The CO2 just builds. And builds. And builds.
By 1986, the deep water of Lake Nyos contained an estimated 300 million cubic metres of dissolved CO2 — roughly five times the volume of the lake itself, compressed into solution at depth.
The Trigger
At approximately 9:30 PM on August 21, 1986, something destabilised the lake. The exact trigger has never been definitively identified. The leading theories include:
- A landslide on the lake's steep inner walls, which would have plunged cold surface water to the depths and disturbed the stratification
- A cold rain on the surface that caused localised cooling and downwelling
- A small volcanic tremor beneath the lake
Whatever the cause, it produced a catastrophic result. Cold, CO2-saturated water from the bottom of the lake was forced upward into zones of lower pressure. As the pressure decreased, the dissolved CO2 came out of solution — like twisting the cap off a shaken soda bottle.
The degassing was self-reinforcing. As bubbles formed and rose, they dragged more deep water upward, which released more CO2, which created more bubbles. The entire deep layer of the lake erupted in a massive, self-sustaining limnic eruption.
An estimated 1.6 million tonnes of CO2 burst from the lake in a single event. The eruption created a fountain of water and gas roughly 80 metres high. The lake turned from blue to a deep, murky red as iron-rich bottom sediments were churned to the surface.
The Invisible Tsunami
Carbon dioxide is roughly 1.5 times heavier than air. The cloud didn't rise — it sank. It poured over the lip of the crater and flowed downhill, following the natural topography like water in a riverbed.
The cloud moved at estimated speeds of 20 to 50 kilometres per hour through the valleys surrounding the lake. In some narrow valleys, where the gas was channelled and concentrated, speeds may have reached 100 km/h.
CO2 at concentrations above 15% causes unconsciousness within one breath and death within minutes. The cloud's concentration at ground level was estimated at 20-30%. People and animals caught in the cloud inhaled a lungful of CO2, lost consciousness instantly, and suffocated before they could move.
The cloud travelled up to 25 kilometres from the lake. It killed everything at ground level. People sleeping in huts, cattle standing in fields, chickens in coops, insects on the ground. The only survivors were those who happened to be on elevated terrain above the cloud's upper boundary — roughly 100 metres above the valley floors.
The Aftermath
When rescue teams arrived on August 22, they encountered a scene unlike anything in modern disaster response.
The villages of Nyos, Subum, and Cha were virtually erased. Bodies lay where they had fallen — in beds, on roads, in the middle of meals. There were no burns, no wounds, no signs of violence. The dead looked as though they had simply fallen asleep.
The survivors were few and disoriented. Some reported smelling a faint sulphurous odour (likely hydrogen sulphide mixed with the CO2). Some recalled hearing a rumbling sound from the direction of the lake. Most remembered nothing — they had gone to sleep and woken up surrounded by the dead.
One survivor, Joseph Nkwain, described waking up hours after the eruption. He could not move or speak. He lay paralysed, surrounded by the bodies of his family, unable to call for help. He survived because his hut was slightly elevated. His wife and children, sleeping in the lower room, did not.
The final death toll: 1,746 people and approximately 3,500 livestock. It remains one of the deadliest natural disasters in African history and the deadliest known limnic eruption ever recorded.
Can It Happen Again?
Yes. And that is the most disturbing part.
Lake Nyos still exists. Volcanic CO2 continues to seep into its depths. Without intervention, the lake will eventually re-saturate and another eruption will occur.
Since 2001, engineers have installed degassing pipes — vertical tubes that extend from the lake's surface to its deep layers. The pipes allow CO2-saturated water to rise to the surface in a controlled, continuous release, gradually reducing the gas concentration in the deep water. The system works, but it's slow, underfunded, and doesn't fully keep pace with the rate of CO2 influx.
Lake Nyos is not alone. Nearby Lake Monoun experienced a smaller limnic eruption in 1984, killing 37 people. And Lake Kivu, on the border of the Democratic Republic of Congo and Rwanda, is far larger and holds an estimated 256 cubic kilometres of dissolved CO2 and methane. Two million people live on its shores.
Scientists are watching. But the lakes are still there. And they're still filling.
The geology of a killer lake
How volcanic CO2 turns a tropical lake into a pressurised time bomb — and what it looks like when that bomb goes off.
The survivors' stories are the most chilling part.
Survivor testimonies
The handful of people who survived Lake Nyos describe waking up surrounded by the dead, unable to understand what had happened. Their accounts are devastating.
But Lake Nyos is not the scariest lake in Africa.
Lake Kivu — the next one?
Lake Kivu holds 300 times more dissolved gas than Lake Nyos did in 1986. Two million people live on its shores. Scientists say the question is not if, but when.
What you now know
- Lake Nyos is a maar lake sitting on an inactive volcano — volcanic CO2 had been dissolving into its deep water under pressure for decades, like a sealed bottle of soda
- The eruption released 1.6 million tonnes of CO2 in minutes, creating a cloud heavier than air that flowed downhill at up to 100 km/h, suffocating everything in its path
- Survivors described waking up paralysed and surrounded by the dead — the CO2 concentration was so high that one breath caused instant unconsciousness
- Lake Kivu, on the Congo-Rwanda border, holds 300 times more dissolved gas than Nyos and has two million people on its shores — degassing efforts are ongoing but incomplete