On 21 August 1986 a huge cloud of carbon dioxide gas was released from Lake Nyos located at 300 metres in the Highlands of Yaounde District of Cameroon. Because carbon dioxide is more dense than air it hugged the ground and flowed down valleys. The cloud travelled as far as 15 miles (25 km) from the lake. It was moving fast enough (as much as 80 kph) to flatten vegetation. 1,700 local people died by suffocation, probably unaware of their plight. Two years earlier 37 people died similarly in a gas release from nearby Lake Monoun
Lake Nyos is in the Oku volcanic field, and is one of several maars produced by one-off explosive events in the recent past. Isotopic analyses of gas remaining dissolved in the lake show that the CO2 is of volcanic origin. The lakes are fed by springs on their beds, which is where the CO2 enters. Being extremely deep (about 200 metres) and with no surface inlet the lake water is strongly stratified, so that CO2-rich water builds up at the bottom. The gas release must have involved an overturn of the stratification, so that dissolved gas came out of solution as pressure decreased. What triggered the overturn is hard to establish, but one possibility is that during August (both catastrophes occurred in that month) cold weather cools surface waters so that they sink. Other possibilities are storms, landslides or earthquakes, but there are no records of any of these preceding either event; they came completely unannounced.
Since 1986, gas levels have built up, and now stand at twice their concentration following the disaster, so danger threatens the local people and their livestock once again. An international team, headed by George Kling a geologist at Michigan University, USA, has devised a means of venting the gas harmlessly. This involves sinking 15 centimetre diameter polyethylene pipes to the lake bed. Once pumping starts, gas bubbles forming as pressure releases will drag the water upwards, as a self-sustaining siphon, similar to the air-lift dredges used in marine archaeology. Four such pipes would rid the lake of its lethal gas content in two years, and even one would reduce the hazard considerably.
Sources: Observer, 20 August 2000, University of Michigan (http://www,biology.lsa.umich.edu/~gwk/research/nyos.html)