On 21 August 1986 a huge cloud of carbon dioxide gas released from Lake Nyos in the Highlands of Yaounde District of Cameroon, killed 1,700 local people by suffocation
Lake Nyos 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, so that CO2-rich water builds up at the bottom. A thermal overturn of Lake Nyos may have caused dissolved gas to come out of solution as pressure decreased.
Since 1986, gas levels have built up, so Lake Nyos once again threatens the local people and their livestock. An international team, headed by George Kling a geologist at Michigan University, USA, has devised a means of venting the gas harmlessly. This involves polyethylene pipes that descend to the lake bed. Once primed by pumping, gas bubbles form as pressure drops. Their rise up the pipe drags more water upwards, as in a soda siphon. Fifteen years after the disaster, the first such siphon began operating with spectacular effects (Jones, N. 2001. The monster in the lake. New Scientist, 24 March 2001, p 36-40). This only keeps pace with addition of CO2 and a full solution requires several siphons.
Some scientists worry that siphoning itself may disturb a precarious balance in the lake, so the French engineers who built it have included sensors and shut-off valves. Not everyone agrees that the 1986 disaster resulted from processes within the deep lake itself. That should have led to a regular succession of gas releases, for which there is little evidence. Landslips or a gaseous eruption might have been the trigger. Reducing dissolved CO2 levels in Lake Nyos and nearby Lake Monoun would seem to lessen risks of a future disaster, but could also lull locals into a false sense of security.