Lake Nyos Disaster
On the night of 21 August 1986, a huge cloud of carbon dioxide burst from Lake Nyos, a crater lake in the northwest of Cameroon, and flowed down into nearby valleys. It killed about 1,700 people, along with thousands of cattle and other animals, mostly in the villages of Nyos, Cha, and Subum. The disaster was a rare natural event known as a limnic eruption, and it led to a new scientific field of study and to engineering efforts to prevent a repeat.
Lake Nyos lies in the crater of a volcano in the Oku Volcanic Field, part of the Cameroon Volcanic Line. The volcano is dormant, but magma deep underground still releases carbon dioxide that seeps into the lake bed and dissolves in the deep water. Because the lake is deep and stratified, with little mixing between layers, the gas can accumulate under pressure, much as it does in a sealed bottle of soda. Few people understood this danger before 1986. Only two years earlier, a smaller gas release at Lake Monoun, also in Cameroon, had killed around 37 people, but its cause had not been firmly established.
The eruption happened late in the evening, and survivors described hearing a rumbling sound and noticing a strange smell. The gas, which is heavier than air, poured over the crater rim and hugged the ground as it travelled down the valleys, reaching some villages as far as roughly 20 kilometres away. People and animals lost consciousness and died of asphyxiation, often in their homes and while asleep. Survivors who woke later in the night or the next day found their neighbors and livestock dead. The trigger remains uncertain. A landslide, a volcanic disturbance, or cold rain or wind disturbing the surface water have all been proposed, but none has been confirmed. Whatever started it, the lake water overturned, and dissolved gas came rapidly out of solution.
Scientific teams soon arrived and concluded that the gas was carbon dioxide rather than a volcanic blast or toxic fumes, as some early theories suggested. Thousands of survivors were displaced, and the government moved many people away from the danger zone, though some later returned.
The disaster prompted major research into gas-rich lakes. Lake Kivu, on the border of Rwanda and the Democratic Republic of the Congo, is another one that has drawn particular attention. At Nyos, a team began installing a degassing pipe in 2001, which lets deep, gas-laden water rise gradually and release its carbon dioxide at the surface in a harmless fountain. Further pipes were added in later years, and they have greatly reduced the gas buildup, though monitoring continues.