On the evening of August 21, 1986, Ephraim Che, a farmer from the highlands of northwest Cameroon, heard a great rumbling in the distance like rolling thunder. Emerging from his hut, he peered down the mountain to the moonlit waters of Lake Nyos, a small but deep body of water nestled in the crater of an extinct volcano. Though a strange white mist now hung over the lake, otherwise everything looked normal. Suddenly overcome by a feeling of illness, Ephraim returned to his hut and went to bed. The following morning, Ephraim rose and began making his way down the mountain. It was not long before he realized something was amiss. The waterfall that cascaded out one end of the lake had suddenly gone dry, while the waters of the lake itself, normally crystal blue, had turned an ugly rust-brown. Then, he noticed the eerie silence: not a single bird was singing, or insect buzzing. Spooked, Ephraim began running down the valley. He soon came upon Halima Suley, a cowherd, shrieking in grief and horror. Scattered around her were the bodies of more than 30 family members and 400 head of cattle. Around 9 PM the previous evening – the same time Ephriam had heard the distant rumble – a great wind had roared down the valley through Suley’s house, causing all the people and animals around her to suddenly lose consciousness. They never woke up. But worse was yet to come. In the nearby village of Nyos, all but two of the 1,000 inhabitants perished in the night, felled where they stood, sat, or lay. Hundreds more were killed in villages up to 25 kilometres away. Those who survived remained unconscious for up to 36 hours before coming to. Many, upon seeing the bodies of their dead loved ones strewn about them, chose to commit suicide. In all, 1,746 people died that night, along with 3,500 head of cattle and countless birds, insects, fish, and other wild animals. It was only days later that wildlife finally returned to the area, when vultures and other scavengers arrived to feast on the corpses.

But what had caused this horrifying catastrophe? Almost immediately, rumours and speculation ran rampant. Some pointed to secret government weapons experiments, toxic waste dumped by corporations, or even ancient legends of evil spirits said to haunt the area. Others suspected toxic gases from a volcanic eruption. But as experts from around the world descended on Cameroon, a more surprising picture began to take shape. The invisible killer, it turned out, was Lake Nyos itself.

Lake Nyos lies in the Oku Volcanic Plain, part of the Cameroon Line of volcanoes stretching 1,600 kilometres northeast from the Gulf of Guinea to Lake Chad. As with all the volcanoes in the Cameroon Line except for Mount Cameroon, the volcanoes of the Oku Massif are now extinct, with two of the craters having filled with water to form Lake Nyos and Lake Monoun. Such crater lakes are typically small in area but extremely deep, with Lake Nyos measuring only 1.9 kilometres long and 1.2 kilometres wide but bottoming out at 200 metres. Below this lies a tall volcanic pipe of porous debris from the last volcanic eruption, as well as a magma chamber 80 kilometres below the surface. While the crater is no longer connected to the magma chamber, gases from the magma can still percolate up through fissures in the rock into the bottom of the lake. And chief among these gases is carbon dioxide. Due to the lake’s great depth, the bottom is extremely cold, dark, and under high pressure – the perfect environment for storing large amounts of gas. Bubbles of carbon dioxide emerging from the rock below grow smaller and smaller as they rise through the water column, never reaching the surface as they are absorbed into the cold, high-pressure water. In most crater lakes this buildup is not an issue, as seasonal temperature fluctuations create convection currents that carry water from the bottom of the lake to the top, safely releasing the trapped gas into the atmosphere. In equatorial regions like Cameroon, however, the temperature is fairly stable year-round, and no such turnover occurs. Instead, carbon dioxide continues to build up for years or even centuries, forming a large supersaturated layer at the bottom of a lake. This gradual buildup creates a ticking time bomb – a bomb which went off on the evening of August 21, 1986.

No-one knows exactly what set off that bomb, with the leading theories being an underwater volcanic eruption, a landslide, or a small earthquake. Whatever the case, that disturbance triggered an upwelling of water from the lake bottom. As this plume rose, the change in pressure and temperature caused the carbon dioxide to bubble out of solution. These bubbles, in turn, entrained more water behind them, creating a positive feedback loop that caused all the carbon dioxide trapped in the lake to bubble out at once – a phenomenon known as a limnic eruption. This eruption unleashed nearly a billion cubic metres of carbon dioxide, creating a tsunami that flattened vegetation around the lakeshore and caused the lake’s water level to drop by a full metre. It also disturbed iron-rich sediments on the lake floor which oxidized as it reached the more oxygen-rich upper layers, producing the dramatic colour change observed by Ephraim Che.

Meanwhile, the carbon dioxide cloud grew to 100 metres high, spilled over the crater rim, and – being heavier than air – flowed down the mountainside and into the twin valleys below at 72 kilometres per hour, smothering everything in its path. The silent, invisible killer engulfed the villages of Nyos, Cha, Fang, Subum, and finally Mashi, travelling 25 kilometres before finally dissipating. Carbon dioxide is odourless and tasteless and produces unconsciousness at concentrations as low as 20%, so most victims died instantly and without warning – though some survivors reported smelling rotten eggs, indicating the presence of sulphur dioxide gas. Of the some 5,000 survivors, most were either standing on higher ground or in poorly-ventilated buildings, and thus inhaled only a sub-lethal concentration of gas. Nonetheless, many remained in comas for days, with 548 being admitted to hospital. Strangely, many of the victims’ bodies were found to be covered in rashes and blisters, which were initially believed to be caused by acidic volcanic gases. Later, it was theorized that the marks were instead bedsores from the victims laying comatose for long periods before finally dying or regaining consciousness. However, these symptoms largely remain a mystery to this day.

While the Lake Nyos disaster may seem like a freak occurrence, such incidents were not new to the region. Just a year before on August 15, 1985, a limnic eruption from the smaller Lake Monoun, 100 kilometres southeast of Nyos, resulted in the deaths of 37 people. Indeed, anthropologists believe that local legends of evil spirits haunting crater lakes, as well as the tradition of certain ethnic groups like the Bafmen of always building houses on high ground, derive from memories of past eruption events. Palaeontologists even theorize that limnic eruptions – albeit on a much, much larger scale – may have been responsible for several mass extinction events, such as the Permian-Triassic extinction of 252 million years ago which saw the disappearance of nearly 80% of all life on earth.

But if such eruptions have happened in the past, then they will inevitably happen again in the future – unless preventative measures are taken. Shortly after the 1986 disaster, an international team of geologists and other experts began examining means of safely releasing the carbon dioxide trapped in Lake Nyos to prevent another tragedy. One proposal involved dropping explosives into the lake to shock the gas out of solution, but this was quickly rejected as it could damage the lake’s thin walls, unleashing a deadly flood on the valleys below. Other proposed solutions included dumping vast quantities of calcium hydroxide or lime into the lake to neutralize the dissolved carbon dioxide, or digging tunnels into the bottom of the lake to drain out the gas-saturated water. However, all were rejected for being too expensive or logistically challenging. Instead, it was decided to sink a pipe to the bottom of the lake to allow the gas to escape gradually.

But while several experimental setups were tested in the 1990s, a permanent solution was slow to materialize, largely due to lack of support from the Cameroonian government and international disaster relief agencies. It wasn’t until 1999 that the U.S. Office of Foreign Disaster Assistance (OFDA) came up with $433,000 to fund the project. In 2001- fifteen years after the disaster – a French engineering team rowed out to the middle of the lake in rafts and sank a 15 centimetre-diameter pipe 200 metres down to the gas-bearing layer. Immediately, a geyser of carbonated water shot out of the pipe at nearly 200 kilometres per hour and rose 50 metres into the air. Nearby Lake Monoun has three such pipes, one installed in 2003 and two in 2006, while solar-powered CO2 detectors installed around both lakes stand ready to warn the locals of any future gas releases. Yet despite these measures, experts remain wary. Some 5,500 tons of carbon dioxide seep into the lake every year, an influx the single vent pipe is just barely able to keep up with. However, no funding is forthcoming to install more pipes. As a result, there is still enough gas at trapped a the bottom of Lake Nyos to cause another major disaster, endangering the lives of more than 10,000 living around the volcano. There is also the flood risk posed by the lake’s fragile walls, while experts have speculated that the venting pipe might actually trigger a future eruption by inducing turbulence in the lake bottom. As of this recording, however, no eruptions have been reported since 1986.

But Lake Nyos and Monoun are not the only crater lakes susceptible to limnic eruption. There are 44 similar lakes in Cameroon’s northwest province alone, and many more around the world, including lake Quilotoa in Ecuador, Lake Ngozi in Tanzania, and Lake Monticchio in Italy. But most worrying of all is Lake Kivu, located on the border of Rwanda and the Democratic Republic of Congo in central Africa. Measuring 2,700 square metres in area and 480 metres deep, Lake Kivu is more than a thousand times larger and twice as deep as lake Nyos, and is thought to contain nearly 256 cubic kilometres of carbon dioxide and 65 cubic kilometres of methane – which, unlike carbon dioxide, is every flammable. Given that nearly 2 million people live near Lake Kivu, a Nyos-style eruption could trigger a natural disaster on a hitherto unheard-of scale. Even more frightening, studies of sediment layers suggest that the lake has already erupted between 7,000 and 8,000 years ago, meaning it might be due for another any day now. As a result, experts are closely monitoring the lake and developing strategies for safely bleeding off the gas – which, as a bonus, could be used to fuel a natural gas power plant. For now, however, the shadow of August 21, 1986 looms over Lake Nyos and other crater lakes across Africa, a constant reminder of how unpredictable – and deadly – nature can be.

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