Lake Nyos belched a CO2 cloud that suffocated 1,746 people overnight

On the night of August 21, 1986, a lake in northwestern Cameroon killed almost everything that breathed for miles around — without a flame, a flood, or a sound. Lake Nyos sits in a volcanic crater, and over decades, magmatic carbon dioxide had been seeping up from below and dissolving into its cold, deep bottom waters, building up like the gas in an unopened soda bottle.

Something — perhaps a landslide, a small earthquake, or simply an unstable column of water — disturbed that pressurized layer. The CO₂ came out of solution all at once, erupting from the lake in a phenomenon geologists call a limnic eruption. A cloud of roughly 100,000 to 300,000 tonnes of carbon dioxide burst out and, being denser than air, hugged the ground and poured downhill.

On 21 August 1986, a lethal carbon dioxide cloud burst from the bottom waters of Lake Nyos, killing 1746 people and more than 3000 livestock.

Carbon dioxide is invisible and odorless. Villagers and animals up to 25 km away simply lost consciousness and asphyxiated where they lay; survivors woke to find their families and herds dead around them. The death toll reached about 1,746 people.

To stop it happening again, engineers later sank pipes into the lake to vent the gas slowly and continuously — defusing one of only a handful of lakes on Earth known to be capable of this.