During the 1980s Lake Monoun and Lake Nyos in Cameroon exploded. Lake Monoun exploded on August 15, 1984 with witnesses describing a loud noise, and a gas cloud that emanated from part of the lake. 37 people were killed, with some confusion as to the causes of skin conditions that were found on some bodies. In August of 1986, 1,700 people were killed at Lake Monoun, when it exploded, much with the same conditions as Lake Nyos.
The culprit apparently is CO2, and the disaster is termed a "limnic eruption". In a limnic eruption a lake which is situated in a volcanic field, will build up carbon dioxide which stratifies in the water, depending on whether there is high turnover of the water and on the water temperature.
Nyos and Monoun are the only two lakes in the world with the limnic explosive potential of the 1980s eruptions. The causes of the gas releases as well as the exact details involved are still clouded in lack of information, as the direct witnesses were killed. Such methods as gas venting have been attempted and seem to have prevented additional buildup of gases.
Both lakes are situated in the Oku Volcanic Field in northwest Cameroon. This unique volcanic field consists of many recent craters and has active volcanic activity. The Oku volcanic field, which includes the Oku volcano, a stratovolcano which peaks at 9880 feet or 3011 meters. Also in the volcanic field are scoria cones and maars, which have become water filled volcanic lakes. The propensity for gases to erupt and to settle in areas in ways that are deadly is high in this region.
Carbon dioxide can escape from the volcanic area into the lake, where it can sit at the cold bottom, building up until a new venting of carbon dioxide exceeds the limits and explodes upward in a massive ball of gas. There are also interactions between the carbon dioxide and the mineral, metal and other components of the water, and even other volcanic gases that are vented, which result in deadly gas clouds which settle to lower areas, killing people and animals.
A third lake, lake Kiva, sits in the Viringu volcanic field of Rwanda, but has a warm bottom and is much deeper than Nyos or Monoun. The potential for a limnic explosion to occur there is based on high levels of Carbon and Methane gases.