On the morning of Friday, August 22, 1986, Joseph Nkwain woke up on the floor of his house in the village of Subum, in northwestern Cameroon. He had been unconscious for about twelve hours. He could not speak — in his own words, his breath would not come out.
The night before, he had heard his daughter making a strange, terrible snoring sound in her sleep. He had tried to cross the room to her bed, and collapsed halfway.
Now his daughter was dead. He went to his neighbours’ houses and found every one of them dead too. He rode toward the nearest town, and later said that on the whole ride, through the village of Nyos, he did not see any sign of any living thing.
The night before, the lake above his village had erupted. Not with lava. Not with fire. With an invisible cloud of gas — and 1,746 people never woke up.
A bottle two hundred metres deep
Lake Nyos looks innocent: a crater lake in the volcanic highlands of Cameroon, about two kilometres long, deep blue, and still. Farmers and herders had lived on its slopes for generations.
But deep beneath Nyos, magma is still at work. Carbon dioxide seeps up through the lake bed and dissolves into the bottom water. And there it stays. The lake is about 200 metres deep, and its water lies in layers that almost never mix. At the bottom, under enormous pressure — roughly 21 times the pressure at the surface — the water can hold a huge amount of dissolved gas, the way an unopened bottle of soda holds its fizz. Decade after decade, the lake quietly charged itself with gas. Nobody measured it. Nobody knew.
Except there had been a warning. Two years earlier, on August 15, 1984, Lake Monoun, another crater lake nearby, released a burst of gas in the night that killed 37 people on a low stretch of road. Scientists investigated and blamed stored gas escaping the water. Nobody realised Nyos held vastly more of it. (Wikipedia — Lake Nyos disaster)
The night of August 21, 1986
At around half past nine that Thursday night, people near the lake heard a rumbling sound. Some went outside to look.
What happened next took minutes. The deep water, saturated with gas, suddenly rose and overturned, and the lake fizzed — violently, like a shaken soda bottle with the cap torn off. A column of water and foam shot about 100 metres into the air. A wave at least 25 metres high swept one shore. By morning, the lake’s level had fallen by about a metre, and its blue water had turned a deep, rusty red, stained by iron-rich water dragged up from the bottom.
But the eruption itself was not the killer. The gas was. Estimates of how much carbon dioxide came out of the lake vary enormously — the landmark 1987 study put it at 100,000 to 300,000 tons; other estimates run past a million tons. Whatever the exact figure, the cloud was enormous, and carbon dioxide is about one and a half times heavier than air. So it did not rise and scatter. It poured downhill, roughly 50 metres thick, moving at 20 to 50 kilometres per hour, and followed the valleys for some 25 kilometres — through Nyos, Kam, Cha, and Subum.
Near the lake, the gas was strong enough to knock a person unconscious within minutes (at concentrations around 10 percent, that is exactly what carbon dioxide does). People died in their sleep, at their cooking fires. In Nyos and Kam, closest to the lake, everyone died except four people — who lived on higher ground.
By morning, 1,746 people and about 3,500 cattle were dead. The villages were untouched. Nothing burned, nothing destroyed. Just silence — and a strange smell survivors described as gunpowder, or rotten eggs, where nothing had burned at all.
The clues that lied
When rescuers and reporters reached the villages, about 36 hours later, one journalist wrote that it looked like the aftermath of a neutron bomb: everything standing, everyone dead. At first, nobody could explain it.
Worse, the evidence pointed the wrong way. Survivors had blister-like wounds that looked like burns — evidence, it seemed, of hot volcanic gas. And survivors swore they had smelled gunpowder and rotten eggs, the signature of sulfur gases.
An international team led by American geologist George Kling tested everything. The lake water held one gas: carbon dioxide. The “burns” were pressure sores, from lying unconscious and unmoving for many hours. And the sulfur smell? At certain levels, carbon dioxide produces sensory hallucinations. The smell had never been there. The gas itself had invented it.
Their report, published in Science in 1987, gave the world a new kind of disaster: a limnic eruption — a lake overturning and dumping its stored gas all at once. A phenomenon science had barely even theorised.
But one question they could not answer has never been answered since: what pulled the trigger? The leading suspect is a landslide — rock crashing into the deep water and starting the overturn. A small volcanic eruption below the lake, a cold rainstorm, a minor tremor have all been proposed. Witnesses felt no earthquake. There were no instruments watching. The official verdict is simply: not known. (ZME Science — How a quiet lake in Africa became one of the deadliest natural disasters nobody saw coming)
Defusing a lake
Understanding what happened was not enough — because the lake was already reloading. Carbon dioxide was still seeping into the deep water, charging Nyos all over again.
The fix, championed by French scientist Michel Halbwachs, sounds like a joke: give the lake a straw.
In 2001, after years of feasibility tests, engineers ran a pipe from a raft down to the deepest, gas-rich water. Start the flow with a pump, and physics takes over. As the deep water rises through the pipe, the pressure falls, the dissolved gas fizzes out, and the fizz itself lifts more water behind it — a self-running fountain, about 50 metres tall, venting the lake’s gas continuously. No power required.
Two more pipes were added in 2011. By 2019, the system had reached a balance: even one pipe, running on its own indefinitely, vents gas as fast as the volcano underneath replaces it. The lake is not cured — the gas still seeps in, and always will. But it is defused.
One worry remains: the natural rock dam holding the lake in its crater is slowly weakening. So Nyos is watched. Permanently. (List25 — Lake Nyos disaster: limnic eruptions and CO2)
The bigger lake
After Nyos, scientists went looking for other lakes like it — and found one that makes Nyos look like a rehearsal.
Lake Kivu, on the border of Rwanda and the Democratic Republic of Congo, is about 2,000 times larger than Nyos. It is also supersaturated, holding an estimated 300 cubic kilometres of carbon dioxide and around 60 cubic kilometres of methane. Geology says lakes like it overturn roughly once every thousand years. About two million people live along its shores.
Along those shores, pockets of carbon dioxide already pool at ground level, silent and invisible. Locals call it mazuku — “evil wind.”
The hopeful twist: Rwanda now draws methane out of Lake Kivu and burns it for electricity. Every unit of gas pulled from the water is danger drained away. The lake is far too big to defuse quickly — the work will take decades. But Nyos taught the world the lesson in the hardest possible way, and for once, the world is acting before the disaster, not after it.
The lake today
Today, Lake Nyos is blue again, and quiet. The fountain from the degassing pipes still plays on its surface — one small, permanent, controlled eruption, so the big one never comes back. And people have returned to the villages on its slopes. They live beside the lake that killed 1,746 of their neighbours. The pipes stand guard.
Nyos gave science a whole new category of disaster. Not the mountain exploding, but the lake exhaling. One of the deadliest natural disasters of the twentieth century made almost no sound, broke almost nothing, and came from something as ordinary as gas dissolved in water.
The gas is still down there, seeping in, molecule by molecule. The difference is that now, so is the straw.
Huh, interesting.