What happened at Lake Nyos in Cameroon in 1986?

On the evening of 21 August 1986, a cloud of carbon dioxide rose from Lake Nyos in northwestern Cameroon and flowed into the valleys below. It killed at least 1,700 people, with commonly cited figures of 1,746, along with roughly 3,500 livestock. The deaths were caused by asphyxiation. This kind of event is called a limnic eruption, in which dissolved gas suddenly comes out of a lake's water. [1] [2] [3]

According to [volcanoes.sdsu.edu](https://volcanoes.sdsu.edu/Nyos.html), the release began at about 9:30 p.m. A mixture of carbon dioxide and water droplets rose from the lake as a jet at roughly 100 kilometers per hour. Carbon dioxide is about 1.5 times as dense as air, so the cloud hugged the ground and ran down the valleys on the north side of the crater. It was about 50 meters thick and stayed lethal for around 23 kilometers, reaching the villages of Nyos, Kam, Cha and Subum. [4]

The Smithsonian's Global Volcanism Program reports that investigators found no evidence of hydrogen cyanide or carbon monoxide, and concluded that victims were suffocated by carbon dioxide. Survivors described smells of gunpowder or rotten eggs, and many had skin burns. The lake level dropped by about one meter, and its upper waters turned red-brown afterward. Some witnesses heard bubbling and low rumblings as the event began. [5]

Lake Nyos sits in a crater in the Oku Volcanic Field. NASA describes carbon dioxide from a pocket of magma deep below seeping up through the crust with groundwater and collecting at the bottom of the lake until the gas becomes too concentrated and bursts out. The 1987 report in Science concluded that the gas was mostly carbon dioxide stored in the lake's deep water, that it came from magmatic sources, and that no significant direct volcanic activity was involved. [1] [3]

The trigger has not been settled. Early on, scientists split between two ideas: a sudden gas eruption through the lake, or a slow buildup in the deep water that was released when the bottom layers overturned. Eos reports that the limnic eruption explanation became the most likely after the lake's bottom water recharged with carbon dioxide in the years afterward. [4] [2]

Among those who favor overturn, the trigger is still uncertain. One proposal is that cool rainwater, possibly pushed to one side of the lake by strong August winds, sank and displaced the bottom water. Rising water would lose pressure, release gas, and speed its own ascent as bubbles formed. The Smithsonian report also lists winds, waves or a landslide as possible sources of turbulence. The SDSU page says there is little or no evidence for a landslide or earthquake, though the Smithsonian notes the nearest seismograph was 150 km away. [4] [5]

The Science authors argued that hazardous lakes could be identified and monitored. According to Eos, artificial degassing of Lake Nyos over about 15 years has made the lake relatively safe. [3] [2]