Lake Nyos is much larger than Lake Monoun and is surrounded by several villages. In 1986 the lake overturned, triggered by a landslide which released hundreds of tons of rocks into the lake. Once the rocks reached the bottom of the lake the gas-rich water exploded upwards in a huge fountain that killed most of the fish in the lake and washed over one side of the crater of the lake. The fountain of water caused a huge tidal wave which ripped all the vegetation from the shore and left only bare rock.
The carbon dioxide released from the lake gradually filled the crater and then spilled over and entered a nearby valley . It is estimated that the cloud would have been travelling at about 60 km/h. The overturn occurred the night before market so there were many more people in the village of Nyos than usual. Everyone in the village was killed apart from six people, and in total about 1,800 people from Nyos and other surrounding villages were killed.
Since the events at Lake Nyos, measures have been taken to prevent further tragedies at the Cameroon lakes. Early warning systems have been installed which measure the degree of gas saturation at various depths. This is done by measuring the ratio of gas pressure to hydrostatic pressure. The current saturation of Lake Nyos is about 70% which is high enough for a large disturbance of the water column to trigger another release of gas.
Work is currently in progress to de-gas Lake Nyos. A pipe has been installed in the centre of the lake held in place by a raft attached to the shores by ropes. The pipe reaches 203m deep (the depth of the lake is 210m) and has valves at 0, 100 and 140m depths. When the valve at the surface is opened CO2 rich water from the bottom is released in a large plume from the top of the pipe due to the pressure difference between the deep water and the atmospheric pressure. If the fountain of water becomes too energetic the valves at 100m or 140m can be opened allowing gas-poor water to enter the pipe and mix with the gas-rich water. This reduces the energy of carbon dioxide coming out of solution and releases strain on the pipe.
The initial removal is greater than the rate of carbon dioxide entering the lake so the amount of CO2 in the lake will gradually decrease. However the removal rate will decrease as the pressure of carbon dioxide in the bottom waters decreases. The amount of carbon dioxide removed by one pipe is insufficient to remove the gas to a safe level within the next 5-10 years. Funding is currently being sought to install a further three or four pipes.
Image of Lake Nyos borrowed without permission from http://volcano.und.nodak.edu
Image of degassing borrowed without permission from http://www.bbc.co.uk/science/horizon/2001/killerlakes.shtml