The Lake Nyos tragedy was a disaster that occurred on August 21, 1986, when a cloud of gas suddenly boiled at Lake Nyos, Cameroon, killing nearly 2000 people within a radius of 20 km. The gas was first speculated to be hydrogen sulphide, but now is believed to be carbon dioxide. Chemical engineers believe that the lake suddenly turned over, bringing dissolved carbon dioxide to the lake surface.
Lake Nyos is a deep lake that is thermally stratified: layers of warm, less dense water near the surface float on the colder, denser water layers near the lake's bottom. Engineers believe that over hundreds of thousands of years, carbon dioxide gas had gotten into the cold water at the lake's bottom and dissolved in great amounts because of the large pressure of CO2 present.
On August 21, water which was saturated with CO2 reached the surface, possibly due to wind or cooling of the lake's surface, and released enormous amounts of the gas which suffocated life throughout the region. Engineers believe this was a chilling illustration of Henry's Law; see also Raoult's law.
International efforts have since installed a pipe running from the surface anchored to a raft that allows the deeper areas of the lake to release their CO2 to the surface in small amounts. It is hoped this will reduce the maximum levels of CO2 in the future, and prevent any possibility of the lake turning over.