In 1986 a quiet lake in Cameroon exhaled a deadly cloud of gas.
Lake Nyos looked peaceful, but its deep water was loaded with dissolved carbon dioxide, like a sealed bottle of fizzy drink. One August night it overturned and released a vast invisible cloud that flowed down the valleys. It killed 1,746 people. Today pipes vent the lake so it cannot happen again.
Lake Nyos sits in a volcanic crater in northwestern Cameroon. Carbon dioxide seeping from magma below dissolved into its deep water, where the weight of the water above kept it in solution. Layers like this are inherently unstable, because the gas's solubility depends on temperature and pressure. If deep water is lifted, the pressure drops and the gas comes out of solution, just as a drink fizzes when opened.
On 21 August 1986, that is what happened in what geologists call a limnic eruption. Bubbles rising from a saturated layer dragged more deep water upward, which released more gas, in a runaway chain reaction. Somewhere between about 100,000 and 300,000 tonnes of carbon dioxide burst out. A column of water and foam rose around 100 metres, and the lake's blue water turned red as iron-rich water from the depths reached the air.
Carbon dioxide is about 50 per cent denser than air, so the cloud sank and poured down the surrounding valleys at up to around 50 kilometres per hour. Concentrated enough to suffocate, it reached villages including Nyos, Kam, Cha and Subum, where many people died in their sleep, along with some 3,500 livestock. Nobody knows what triggered the overturn: suspects include a landslide, a small volcanic eruption or cold rain, and some scientists argue no trigger was needed at all.
It is one of only two recorded limnic eruptions, the other being at nearby Lake Monoun two years earlier. Engineers have since installed degassing pipes that lift bottom water to the surface, where the escaping gas keeps the flow going on its own. By 2019 the lake had reached a steady, safe state. Scientists also studied far larger Lake Kivu for similar risks, though a 2020 study found its danger did not seem to be rising.
Source: Wikipedia — Lake Nyos disaster · Text summarised from Wikipedia (CC BY-SA 4.0)
