Lake Nyos: The Lake That Released a Deadly Cloud in One Night

Lake Nyos looks like any quiet mountain lake. It sits in the crater of an old volcano in the highlands of northwest Cameroon, surrounded by green hills and small farming villages. But deep beneath the lake, carbon dioxide seeps up from magma far below, and it dissolves into the cold bottom water. Because the lake's layers rarely mix, that gas builds up for years, held in place by the weight of the water above it. It works like a sealed bottle of soda. On the night of August 21, 1986, the bottle opened. Something disturbed the lake's deep water. Scientists still don't know exactly what. As the bottom water rose, the pressure dropped, and the dissolved gas came out of solution all at once. The lake released an estimated 100,000 to 300,000 tons of carbon dioxide in a single burst. Carbon dioxide is invisible and heavier than air. Instead of rising, the cloud poured over the crater rim and flowed down the valleys, displacing the air in its path. It swept through the villages of Nyos, Kam, Cha and Subum while most people were asleep. By morning, 1,746 people were dead, along with about 3,500 cattle and other livestock. In the village of Nyos, almost no one survived. It took scientists time to understand what had happened. The event is called a limnic eruption, and it is extremely rare. The only other one recorded in modern history happened two years earlier at Lake Monoun, about 100 km away, and killed 37 people. The danger at Nyos didn't end in 1986, because the gas kept building. So researchers came up with a way to vent it safely. In 2001, a team installed a pipe running from the deep water to the surface. Once gas-rich water starts rising through the pipe, it keeps flowing on its own, spraying a plume of water into the air and releasing the gas slowly. Two more pipes were added in 2011, and the concentration of gas in the lake has dropped substantially since. The lake is still there, still quiet, and still releasing its gas, now a little at a time.