The eruption of August 21, 1986
On the evening of August 21, 1986, farmers near Lake Nyos heard a rumbling sound. A fountain of water and foam shot over 100 meters into the air, creating a wave at least 25 meters high that flattened trees on the shore. This event triggered the sudden release of an immense cloud of carbon dioxide. Estimates suggest between 100,000 and 300,000 tons of CO₂ gas erupted from the lake in a matter of minutes.
The gas cloud, about 1.2 cubic kilometers in volume, initially rose at nearly 100 kilometers per hour. Because CO₂ is about 1.5 times denser than air, the invisible, odorless cloud sank and hugged the ground, flowing down valleys at speeds of 20 to 50 kilometers per hour. It spread out over a 25-kilometer radius, silently engulfing the villages of Nyos, Kam, Cha, and Subum. The gas displaced the oxygen, causing rapid asphyxiation. In total, 1,746 people and around 3,500 livestock perished. Survivors reported feeling a warm sensation and smelling a sulfurous odor before losing consciousness, some for as long as 36 hours. After the event, the blue water of the lake had turned a deep red, a result of iron-filled water from the bottom rising to the surface and oxidizing.
A rare geological threat and its solution
The Lake Nyos disaster is a rare type of natural hazard known as a limnic eruption, or "lake overturn". The lake sits in a maar, a crater formed by a volcanic explosion roughly 400 years ago, above a pocket of magma 80 kilometers deep. This magma constantly leaks CO₂, which percolates up and dissolves under high pressure in the cold, deep waters of the 208-meter-deep lake. Over time, the bottom layers of the lake became supersaturated with the gas.
The 1986 eruption was likely triggered by a landslide, though an earthquake or even heavy rain could have been the cause. This disturbance caused the deep, gas-rich water to rise. As it rose, the pressure decreased, allowing the dissolved CO₂ to come out of solution explosively, like opening a shaken bottle of soda. This event was not without warning; a smaller limnic eruption at Lake Monoun, about 100 kilometers away, killed 37 people in 1984.
To prevent a recurrence, an international team of scientists devised a solution to safely remove the gas. In 2001, the first degassing pipe was installed. This system uses a pump to initially draw the deep, CO₂-rich water up the pipe. Once the water rises to a depth where the pressure is low enough for the CO₂ to form bubbles, the process becomes a self-sustaining siphon, pulling more deep water up and releasing the gas in a controlled fountain at the surface. Two more pipes were added in 2011 to increase the rate of gas removal.