There are three lakes in the world which exist under a very unusual and dangerous set of conditions. They are all in areas of volcanic activity. Springs feed into the bottom of these lakes, containing huge amounts of dissolved carbon dioxide, released by magma below the earth's surface. When triggered the lower waters of these lakes can release huge clouds of carbon dioxide gas in an eruption from the lake caused by gas bubbles rising rapidly through the water. This is known as "lake overturn" as the lake is literally overturned as carbon dioxide from the bottom of the lake is released pushing water up with it. The only three lakes which contain such high amounts of CO2 are lakes Monoun, Nyos and Kivu in Africa.
Lake overturn is a relatively newly discovered geological phenomenon. It first came to light in 1984 in Cameroon when 37 people dropped dead from suffocation near Lake Monoun. Two years later the same thing happened but on a much larger scale when 1800 people were killed, apparently by suffocation by Lake Nyos also in Cameroon. Strange clouds of white gas were seen in the areas where people were killed. Geological experts concluded that the tragedies had been caused by clouds of carbon dioxide gas released from the lake, silently suffocating animals and humans in the area.
Once the cause of the deaths at Lakes Nyos and Monoun was discovered other lakes were investigated to see if there were any possibilities of the same thing happening elsewhere. The only other lake found with high enough amounts of carbon dioxide present was Lake Kivu in Rwanda, East Africa. There have been no releases of carbon dioxide from the lake in living memory but the high amount of volcanic activity in the region means that the area is very unstable and it is likely an overturn could be triggered.
Carbon Dioxide - A gas at room temperature, harmless in low concentrations. Molecular formula CO2
Image borrowed with permission from http://perso.wanadoo.fr/mhalb/nyos/
Last updated 19th June 2002