Lake Nyos is in western Cameroon, near the Nigerian border. It is one of several crater lakes within the Oku volcanic field.
On August 12, 1986, a cloud rose rapidly from the floor of Lake Nyos. The expulsion was neither lava nor volcanic ash, but carbon dioxide. It rose as a jet, then, heavier than air, the 50-meter-thick cloud of CO2 began to sink and spread out along the ground.
The sudden heavy concentration of carbon dioxide killed about 1700 people. Thousands of cattle in the region died of asphyxiation. Birds and other animals suffocated as well.
As Maureen K. Fleury explains, fifty miles below Lake Nyos lies a pool of magma. It is constantly bubbling, releasing CO2 and other gases that escape upward until they find their way into the bottom of the deep cold lake.
In 1986, the water in the bottom of the lake reached saturation point; the explosion was a massive CO2 release.
Since 1986, the lake has been monitored and scientists have devised gas release mechanisms they hope will prevent such a disaster from occurring again.