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People who live near volcanoes have visible indications of the presence of the potential danger: a smoking mountain or rumbling earthquakes. But sometimes the danger is hidden. Such was the case at Lake Nyos, a crater lake that is part of the active Oku chain of volcanoes in northwestern Cameroon, Africa. The volcanoes are part of a failed rift valley, formed when South America split from Africa.
At about 9:30 p.m. on August 21, 1986, a cloud of carbon dioxide (CO₂) mixed with water droplets rose from the depths of Lake Nyos. This cloud formed a fog that rolled down into the nearby valleys, suffocating 1,700 people as they slept and killing countless numbers of animals: cattle, birds, wild animals. The people who survived reported waking to silence all around, with people dead in their homes and animals lying everywhere.
How did this happen at a seemingly quiet and peaceful lake? It turns out that CO₂ from volcanic activity accumulates at the bottom of Lake Nyos and another nearby lake, Monoun. Unlike most lakes, the water in these two lakes does not turn over regularly. This keeps the CO₂ at the bottom, where it reaches very high concentrations. In 1986 Lake Nyos essentially burped, releasing the gas, with tragic consequences. Since that terrible event, the survivors have been relocated farther from the lake. In addition, scientists have installed plastic pipes and pumps into Nyos and Monoun, to release the CO₂ from the depths and prevent another such tragedy.