The silent killer
On the evening of August 21, 1986, a rumbling sound emerged from Lake Nyos, a crater lake in northwestern Cameroon. Soon after, a massive cloud of gas, invisible and silent, burst from the lake. This was not a typical volcanic eruption with lava and ash, and was a rare phenomenon known as a limnic eruption. An estimated 100,000 to 300,000 tons of carbon dioxide, which had been trapped in the lake's deep water, were suddenly released.
The resulting gas cloud, being 1.5 times denser than air, hugged the ground and flowed down nearby valleys. It moved at speeds of 20 to 50 kilometers per hour, displacing the breathable air in its path. The cloud, about 50 meters thick, silently enveloped the villages of Nyos, Kam, Cha, and Subum. In all, 1,746 people and about 3,500 livestock suffocated, many in their sleep. Survivors reported smelling rotten eggs or gunpowder, suggesting other gases such as hydrogen sulfide may have been present, though the primary agent was CO2. The normally blue lake turned a deep red color, the result of iron-rich water from the bottom being churned up to the surface and oxidizing.
A geological trap
Lake Nyos fills a maar crater, formed by a violent, gas-rich explosion roughly 400 years ago. It is a deep lake, measuring 208 meters at its maximum depth. Deep beneath the lake, a pocket of magma, a remnant of the region's volcanic past, continuously leaks carbon dioxide into the groundwater that feeds the lake from below.
Ordinarily, lakes mix, bringing deep water to the surface where dissolved gases can escape. Lake Nyos, however, is thermally stratified; layers of colder, denser water at the bottom do not mix with the warmer, lighter water at the surface. The immense pressure at the lake's 200-meter depth allows vast quantities of CO2 to dissolve into this bottom layer, like a highly pressurized bottle of soda. Before the 1986 event, the total gas pressure in the deep water reached as high as 15.6 bar. This saturation is unstable. a trigger, perhaps a landslide, an internal wave, or even heavy rainfall cooling the surface—can disturb the layers. This pushes the gas-rich water upward into lower pressure zones, where the CO2 rapidly comes out of solution. This process creates a chain reaction, leading to a massive, violent degassing event. A smaller, similar event occurred two years earlier, on August 15, 1984, at Lake Monoun, about 95 kilometers away, killing 37 people.
To prevent another catastrophe, an international effort installed a degassing system. The first pipe was placed in 2001, with two more added in 2011. These pipes act like siphons, bringing the CO2-saturated water from the lakebed to the surface. As the water rises, the pressure drops, and the dissolved CO2 fizzes out, creating a self-sustaining fountain that safely vents the gas into the atmosphere.