August 21, 1986 was a busy market day in the village of Lower Nyos (Cameroon) and most people that evening went to bed early. At 9:30 p.m. a strange sound, like a distant explosion, was heard and suddenly people and animals tumbled onto the ground. When the few survivors awoke the next morning, they discovered that more than 1.700 people died. The huts and gardens were untouched, but everywhere they found corpses of humans and carcasses of animals - not even insects had been spared by the unseen killer.
Fig.1. Some of the more than 3.000 carcasses of animals as found some days after August 21. The bodies showed no injuries, photo by geologist Jack Lockwood/ US Geological Survey (image in public domain).
Soon the killer was identified as a strange volcanic phenomenon - 1,6 million tons of carbon dioxide had suddenly erupted from the nearby crater Lake of Nyos. As carbon dioxide is heavier than the surrounding air, the gas formed a 50m thick asphyxiating layer above the entire region.
Lake Nyos is located in an ancient volcanic caldera, more than 200m deep and delimitated by steep cliffs. Water mixing is very limited and gases emanating from the ground (of volcanic origin) dissolve and become concentrated in the deeper water. The tropical temperatures form a sort of cap of warm water (less dense) above this cooler water, oversaturated with possibly lethal gases.
Geologists are not sure why this cap suddenly failed. Maybe an earthquake or volcanic eruption on the bottom of the lake disrupted the water stratification. The days before the catastrophe were rainy. It is possible that the rainfall cooled the surface of the lake until an intermixing with the underlying toxic water occurred. Possibly one or more landslides felt into the lake, disrupting the unstable water stratification.
The particular geological and geographical conditions forming "lethal lakes" are found at two other African lakes. August 15, 1984 an explosion, probably caused by a sudden gas escape, killed 37 people at Lake Manoun. In Lake Kivu the concentration of gases - in part of volcanic, in part of bacterial origin - is also extraordinarily high.
To prevent future catastrophic degassing of the lakes degassing pipes - bringing slowly the oversaturated water from the depth to the surface - were installed, however this solution works only for smaller lakes - the threat from Lake Kivu remains a geological possibility.
DECKER, R. & DECKER, B. (1991): Mountains of Fire: The Nature of Volcanoes. Cambridge University Press. Cambridge: 243