Crater lakes that accumulate dissolved CO2 until catastrophic release. Lake Nyos belched carbon dioxide in 1986 that silently suffocated 1,700 people within 25 kilometers. Pipes now continuously degass the lake to prevent another limnic eruption.
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.
💡Fun Facts
Lake Nyos is one of three known lakes in the world that are saturated with carbon dioxide and prone to limnic eruptions. The others are Lake Monoun, also in Cameroon, and Lake Kivu on the border of the Democratic Republic of Congo and Rwanda.
After the eruption, the water level of Lake Nyos dropped by about a meter, and a wave at least 25 meters high swept the shoreline.
The gas cloud rose initially at nearly 100 kilometers per hour before descending on the surrounding villages.
Some survivors remained unconscious for as long as 36 hours after being enveloped by the gas cloud.
The area is a military zone and access is not on a fixed schedule. Visits should be arranged in advance.
Admission
Permission is required from a nearby military post. Costs are associated with hiring a guide and vehicle.
Accessibility
The region is remote, and roads are underdeveloped and often impassable, especially during the rainy season. A 4x4 vehicle is necessary. Travel is recommended only during the dry season (December to May).