21 August 1986 was a market day in the hills of northwest Cameroon. People in the villages around Lake Nyos had spent the day trading, walking and talking, and by nightfall most of them were tired. Many went to bed early.
At around 9:30 that evening, some of them heard a sound. Survivors later described it as a distant rumble, like thunder or a far-off explosion.
By morning, 1,746 people were dead. So were around 3,500 cattle, along with birds, insects and almost every living thing for miles. There was no fire, no flood and no collapsed buildings. Food was still on tables. Lamps had burned out on their own.
Whatever had happened, it had left almost no trace.
A beautiful, dangerous lake
Lake Nyos sits in the crater of an old volcano, more than 200 metres deep and ringed by steep cliffs. From above, it looks peaceful, the kind of place you’d stop to take a photo.
But the volcano beneath it isn’t entirely asleep. For centuries, carbon dioxide had been seeping up from deep underground and dissolving into the bottom of the lake. Most lakes mix their water as the seasons change, which lets gases escape slowly. Nyos didn’t. Its waters sat in still layers, with warmer water on top acting like a lid on the cold, gas-heavy water below.
Think of a bottle of fizzy drink that has been shaken and left unopened for hundreds of years. That was Lake Nyos.
The cap comes off
Nobody knows exactly what disturbed the lake that night. It might have been a landslide, a small volcanic event, or something as ordinary as cold rain sinking into one side of the water. Whatever it was, it pushed some of the deep water upward.
As that water rose, the pressure on it dropped and the dissolved gas burst free, which pulled more water up behind it. Within moments, the whole lake was erupting. Scientists call it a limnic eruption, and it is one of the rarest natural disasters on Earth.
Somewhere between 100,000 and 300,000 tonnes of carbon dioxide (some estimates go far higher) shot out of the lake. The cloud rose fast, then did what carbon dioxide always does: it sank. Heavier than air, it spilled over the lip of the crater and rolled down into the valleys below as a layer of gas tens of metres thick.
It flowed into the villages of Nyos, Kam, Cha and Subum, pushing out the breathable air as it went. Carbon dioxide has no colour and, at these levels, gives little warning. People lost consciousness in seconds, many of them in their sleep. The gas reached as far as 25 kilometres from the lake.
The morning after
Survivors who woke up the next day found a silent landscape. Some had been unconscious for hours. Many described strange smells, though scientists later concluded these were likely hallucinations caused by the gas itself.
The lake had changed too. Its blue water had turned a murky red as iron-rich water from the depths reached the surface. The shoreline vegetation was flattened, as if something huge had surged over it.
At first, rumours filled the gap: poisoned water, a secret weapon test, an angry spirit in the lake. It took scientists weeks to piece together the truth, and it was stranger than any of the rumours.
It had happened before
The most unsettling detail? Nyos wasn’t the first. Two years earlier, in August 1984, a similar gas release at Lake Monoun, about 100 kilometres away, killed 37 people. At the time, it was a puzzle few outside Cameroon paid attention to.
Monoun and Nyos remain the only two limnic eruptions ever recorded. Both happened in the same region, in the same month, two years apart.
Defusing a lake
The solution scientists came up with sounds almost too simple. In 2001, they lowered a pipe from a raft into the deepest part of Lake Nyos. Once a little water was pumped up, the gas expanding inside the pipe kept the flow going by itself, sending a fountain of water and gas into the air and slowly releasing the pressure.
Two more pipes were added in 2011. By 2019, researchers found the lake’s gas levels had settled into a steady, much safer state. Today, the fountains still hiss quietly in the middle of the water, and an alarm system watches for rising carbon dioxide levels.
Lake Nyos is blue again. Some people have returned to the area. But for those who lived through that night, the lake will never look quite ordinary.
I told this story in full on my YouTube channel. If you’d rather watch than read, head over to The Strange Files.