In 1986, a Cameroonian lake released a CO2 cloud that killed 1,746 people overnight

On the night of 21 August 1986, Lake Nyos in northwestern Cameroon released between 100,000 and 300,000 tons of carbon dioxide in a single outgassing event. The cloud rose at nearly 100 km/h, then sank — carbon dioxide is 50 percent more dense than air — and moved downvalley at 20 to 50 kilometers per hour. Within 25 kilometers, the gas killed 1,746 people and 3,500 livestock in the villages of Nyos, Kam, Cha, and Subum. Reporters described the scene as "looking like the aftermath of a neutron bomb."

Joseph Nkwain, a survivor from Subum, described waking hours later: "I went into my daughter's bed, thinking that she was still sleeping. I slept till it was 4:30 in the afternoon … I got my motorcycle … As I rode … through Nyos I didn't see any sign of any living thing."

What triggered the eruption remains unknown. A degassing system was installed at the lake beginning in 2001; by 2019 it had reached a self-sustaining state that requires no external power to maintain. Lake Kivu in the Democratic Republic of Congo, 2,000 times larger than Lake Nyos, may hold similar risk.