Lake Nyos, in north-west Cameroon, fills an old volcanic crater. From the shore it looks calm. But for years its deep water held a vast amount of dissolved carbon dioxide.
On the night of 21 August 1986, much of that gas came out in one sudden release.
The gas was hiding in the water.
Carbon dioxide from deep underground seeped into the bottom of the lake in spring water. The lake is 208 metres (682 ft) deep, and the weight of water above keeps far more gas dissolved down there than could stay in water at the surface.
The water also rarely mixed. The deep layer was denser than the water above it, partly because dissolved CO₂ makes water heavier, so the layers stayed apart and the gas kept building up. It takes an unusual lake to do this.
Once it started rising, the release fed itself.
What set it off is still debated. The US investigators called the trigger undetermined; a landslide into the lake and a wave between its layers are among the ideas.
The runaway part is better understood. As deep water rose, pressure fell and CO₂ came out of solution as bubbles. The bubbly water was lighter, so it rose faster, drawing more water with it. Scientists call this a lake, or limnic, eruption. Nothing was burning.
The CO₂ came from a deep magmatic source, but investigators found no significant volcanic activity: the deep water was no warmer than in other tropical lakes.
The cloud followed the valleys.
Carbon dioxide is about one and a half times as dense as air. So the cloud spilled over the crater rim and flowed downhill into villages where people were going about their evening. At least 1,700 people died, asphyxiated by the carbon dioxide.
Two years earlier, a similar, smaller gas disaster at Lake Monoun, also in Cameroon, had killed 37 people.
Engineers began letting the gas out on purpose.
In 2001 a permanent pipe, hanging from a raft, began drawing water from about 200 metres (660 ft) down. Once the flow is started, falling pressure lets the CO₂ bubble out, and the bubbles lift the water faster, pulling more up from below. The USGS compares it to a shaken bottle of champagne. No pump is needed to keep it going.
The physics that made the 1986 release so sudden was turned into a slow, controlled one.
The next time a fizzy drink foams as you open it, that's the same physics, on a far smaller and safer scale.
Sources checked 23 October 2026·5 sources ↓