A peaceful village, a sudden catastrophe
On the night of 21 August 1986, the village of Nyos in north-west Cameroon was a peaceful place. Its people had gone about their work as usual, some in the fields, others leading cattle to pasture. The village sat beside a gaping hole almost 210 metres deep, at an altitude of 1,091 metres — an old volcanic crater filled with stagnant water. Locals were proud of the site, which was listed as one of the region's must-see tourist attractions. No one suspected that tragedy was about to visit the small community as they went to bed.
Sometime between 9pm and 10pm that evening, witnesses reported hearing a noise like a landslide. Scientists believe it was most likely a minor earthquake, identified as the starting point of the disaster. The lake's deepest, gas-rich layers of water rose rapidly to the surface, and carbon dioxide was released in a massive burst, creating a geyser and an enormous white cloud.
Heavier than air, the gas cloud fell back down and flowed along the mountainside like an invisible avalanche, following nearby valleys at 60-100 km/h. As it moved, the carbon dioxide drove out all the oxygen in its path. Sleeping villagers and animals lost consciousness and died within seconds. Victims were recorded in the villages of Nyos, Subum and Cha, numbering in the hundreds.
A heavy blanket of silence
On the morning of 22 August, survivors from villages on higher ground awoke to scenes of death and devastation. A reporter for the state broadcaster, Cameroon Radio Television Corporation, sent to the area, described "a heavy blanket of silence" — as if all life had come to a halt. Television cameras captured images of cattle, insects, birds and people lying motionless everywhere. The final death toll stood at 1,746 people and 3,500 animals.
A scientific puzzle and its solution
In the years that followed, scientists pieced together what had happened. Lake Nyos had become supersaturated with carbon dioxide seeping from volcanic activity below. The exact trigger — possibly the minor earthquake, or perhaps a landslide — caused the deep, gas-rich water to overturn, releasing the gas in a sudden, catastrophic burst. This type of event, now known as a limnic eruption, was poorly understood at the time.
Forty years on, experts agree the lake no longer poses the danger it did in 1986. The risk has been greatly reduced through artificial removal of the gas. In 2001, the first de-gassing pipe was installed in Lake Nyos, reaching a depth of around 203 metres. Scientific measurements show the concentration of carbon dioxide in the lake has fallen considerably since then, though monitoring remains essential.
Research on Lake Nyos has also improved scientists' understanding of similar lakes, including Cameroon's Lake Monoun and Lake Kivu on the DRC-Rwanda border. The lessons learned from the tragedy have helped develop methods to prevent future disasters, including the installation of de-gassing pipes in other at-risk lakes.