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 exploding lakes to be saturated with carbon dioxide in this way, the others being Lake Monoun, 100 km (62 mi) away SSE, and Lake Kivu in Rwanda. On August 21, 1986, possibly triggered by a landslide, Lake Nyos suddenly emitted a large cloud of CO2, which suffocated 1,700 people and 3,500 livestock in nearby towns and villiges.
Though not completely unprecedented, it was the first known large-scale asphyxiation caused by a natural event. To prevent a recurrence, 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 (Lake Nyos) also poses a threat due to its weakening natural wall. A geological tremor could cause this natural dike to give way, allowing water to rush into downstream villages all the way into Nigeria and allowing much carbon dioxide to escape.
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