Lake Nyos is an active crater lake that formed by an eruption about 5 centuries ago.
A pocket of magma lies beneath the lake and leaks carbon dioxide (CO2) into the water, changing it into carbonic acid. Nyos is one of only three known lakes to be saturated with carbon dioxide in this way, the others being Lake Monoun, at a distance of 100 km SSE, and Lake Kivu in Rwanda. On 21 August 1986, possibly triggered by a landslide, the lake suddenly emitted a large cloud of CO2, which suffocated 1,700 people and 3,500 livestock in nearby villages. Though not completely unprecedented, it was the first known large-scale asphyxiation caused by a natural event. To prevent a repetition, a degassing tube that syphons water from the bottom layers of water to the top allowing the carbon dioxide to leak in safe quantities was installed in 2001, though additional tubes are needed to make the lake safe.
Today, the lake also poses a threat due to its weakening natural wall. A geological tremor could cause this dike to give way, allowing water to rush into downstream villages all the way into Nigeria.