How a Lake Killed 1,746 People in One Night

The Mechanism

Lake Nyos sits in the crater of a dormant volcano in the Oku Volcanic Field in the Northwest Region of Cameroon, about 315 kilometres north-west of the capital, Yaounde. The crater is half a kilometre across and the lake at its centre is 208 metres deep - deep enough that the water at the bottom is held under a pressure of about 21 atmospheres. Water under pressure can hold a lot of dissolved gas: at 200 metres depth, water can hold roughly twenty times more carbon dioxide than at the surface. Over hundreds or thousands of years, magmatic CO2 from the volcanic plumbing system beneath the lake had been slowly seeping upward into the cold dense bottom water of the lake and dissolving there. The lake was thermally stratified: a warm, low-density, gas-poor surface layer sat on top of a cold, dense, gas-rich bottom layer, with a sharp thermocline between them. By August 1986 the bottom water was near saturation - it was holding about as much dissolved CO2 as it physically could at that pressure. The system was metastable, like an unopened bottle of carbonated water: the CO2 was perfectly stable as long as no one disturbed it, but any large enough disturbance could trigger a runaway release. We do not know exactly what the trigger was on the night of 21 August 1986 - candidates include a landslide on the steep crater wall, a small earthquake, or a cold-front-driven internal wave in the lake. Whatever the cause, a parcel of saturated water from the bottom was lifted high enough toward the surface to start exsolving CO2 bubbles. The bubbles lowered its density. The lower density made it rise faster. The faster rise meant more exsolution. The chain reaction emptied the lake of its dissolved gas inventory in about an hour. The released CO2 cloud shot upward at nearly 100 kilometres an hour at first, then - being 1.5 times denser than air - collapsed back down to ground level and started flowing downhill out of the crater. It moved along the Subum and Cha river valleys at about 20 to 50 kilometres an hour in a layer roughly 50 metres thick. It hugged the topography. It displaced all breathable air. In the villages of Nyos, Cha, Subum, and Fang, within 25 kilometres of the lake, 1,746 people were killed in their sleep along with 3,500 livestock and effectively every bird in every tree. Carbon dioxide at concentrations above about 30 percent in the air causes loss of consciousness within two breaths and death within minutes. Survivors who had been sleeping uphill of the gas layer reported hearing nothing at all between bedtime and dawn. By the time the first scientific teams reached Lake Nyos two days later, the surface of the lake was a striking rust-red colour - turbid with iron-bearing bottom water that had been dragged to the surface by the eruption - and the lake had been almost completely de-stratified. The mechanism was christened a "limnic eruption" only after this event. A smaller limnic eruption had happened two years earlier, on 15 August 1984, at nearby Lake Monoun, killing 37 people; at the time it had not been understood and was assumed to be a tribal-warfare incident. In 1990 a French physicist named Michel Halbwachs at the University of Savoie proposed a way to make Lake Nyos safe. Sink a 200-metre polyethylene pipe straight down into the lake, anchored to a small floating raft at the surface. Prime the pipe once by pumping surface water down it. Once primed, the gas-saturated bottom water starts up the pipe; as it rises, the hydrostatic pressure drops, CO2 exsolves into bubbles, the bubbles lower the column density, and the flow accelerates. The result is a self-sustaining gas-water fountain shooting 50 metres into the air above the lake, releasing dissolved CO2 to the atmosphere at a controlled rate of about 10,000 tonnes a year. The first pipe was installed at Lake Nyos in January 2001. Two more were added in 2011. By 2026 the three pipes have removed about 95 percent of the standing gas inventory, and the lake is below the saturation threshold. It cannot erupt again until it recharges. As of 2026, only three lakes on Earth are known to be capable of a limnic eruption: Lake Monoun (Cameroon, 37 deaths in 1984), Lake Nyos (Cameroon, 1,746 deaths in 1986), and Lake Kivu on the Rwanda-DRC border. Lake Kivu is by far the largest. Its bottom waters hold roughly 256 cubic kilometres of dissolved carbon dioxide and 65 cubic kilometres of dissolved methane. On its northern shore is the city of Goma. The population of Goma is approximately 2 million people. Lake Kivu has no installed degassing system.