In August, 1986, Lake Nyos burped. This is quite unusual for lakes and is due to a unique conflux of geologic factors.
For one thing, Lake Nyos sits on top of a volcanic hotspot, formed in the crater of a four-hundred year old explosion. The rocks here are actually still active, although you wouldn’t guess it from the surface. All the activity happens far below, where a still-living magma chamber vents carbon dioxide, hydrogen, and sulfur gas, which leach upwards through the rock. However, instead of venting out of the ground, they meet the waters of Nyos and dissolve in it.
Normally, the gases would continue upwards and emerge harmlessly from the surface of the water, but Nyos is different. The lake is so deep that different layers of water have formed: a layer of warm, less dense water that floats on top of a colder, denser one; effectively capping the gas-rich waters below. This prevents the gas from escaping into the atmosphere. So instead it stays in the water and slowly builds over time. Eventually the water cannot hold anymore and the whole system reaches a critical supersaturated state. At this point, any upset to the lake can cause the gases to violently erupt upwards.
In 1986 such an event occurred. The exact trigger of the event is unknown, but in one huge eruption 1.6 million tons of carbon dioxide suddenly rushed up from the waters of Lake Nyos. This formed a fountain 300 feet tall and created a 80 foot wave that rushed outwards. But the wave, although destructive to those living nearby, wasn’t the real threat. Instead the carbon dioxide gas, heavier than the surrounding atmosphere, settled back down over the lakes surface, hugging the ground and creating a mass of deadly, unbreathable air known as a mazuku.
To the north of the lake were two valleys, each of which contained a handful of villages. These sloped down away from the lake and created a natural funnel for the gas, which quickly started to flow downhill. The invisible cloud would have moved at up to thirty miles an hour, displacing the oxygen-rich air surrounding the villages and suffocating any caught in it’s path. Seventeen-hundred people died. And even if the victims survived the gas, they would have had to deal with numerous deleterious effects and gruesome scenes as they evacuated. One local, Joseph Nkwain, describes what he saw:
I could not speak. I became unconscious. I could not open my mouth because then I smelled something terrible . . . I heard my daughter snoring in a terrible way, very abnormal . . . When crossing to my daughter’s bed . . . I collapsed and fell. I was there till nine o’clock in the (Friday) morning . . . until a friend of mine came and knocked at my door . . . I was surprised to see that my trousers were red, had some stains like honey. I saw some . . . starchy mess on my body. My arms had some wounds . . . I didn’t really know how I got these wounds . . .I opened the door . . . I wanted to speak, my breath would not come out . . . My daughter was already dead . . . I went into my daughter’s bed, thinking that she was still sleeping. I slept till it was 4:30 p.m. in the afternoon . . . on Friday. (Then) I managed to go over to my neighbors’ houses. They were all dead . . . I decided to leave . . . . (because) most of my family was in Wum . . . I got my motorcycle . . . A friend whose father had died left with me (for) Wum . . . As I rode . . . through Nyos I didn’t see any sign of any living thing . . . (When I got to Wum), I was unable to walk, even to talk . . . my body was completely weak.
Since then the international community has taken an interest in monitoring the lake. Scientists and government officials are carefully measuring the levels in not only Nyos but also two other lakes, Lake Monoun and Lake Kivu, that also possess similar outgassing properties. To release the pent up gas, deep siphons have been installed on floating platforms, effectively creating a straw down to the lower colder layers of water. The resulting fountain allows the lake to relieve itself in a more controllable (and less deadly) manner. More are planned.