Lake That Explodes Without Warning
In this post
A quiet Thursday evening
The evening of 21 August 1986 settled over the hills of northwestern Cameroon the way thousands of evenings had settled before it. Farmers came home from their fields. Cattle were brought in for the night. Cooking fires sent thin lines of smoke into the cooling air, and families in the villages of Nyos, Kam, Cha and Subum ate, talked, and one by one went to sleep.
Above the villages, resting in the crater of an old volcano, lay Lake Nyos. Its water was calm and blue. Children grew up beside it, and nobody in the valley had any reason to be afraid of it. Nothing about that peaceful evening suggested that the lake had been quietly preparing a disaster for longer than anyone alive could remember.
The secret the deep water was keeping
Lake Nyos fills a crater that was formed by volcanic activity roughly four hundred years ago, and it sits along a chain of volcanoes known as the Cameroon Volcanic Line. Deep beneath the lake, volcanic gases are still slowly rising from the earth, and one of those gases is carbon dioxide.
Carbon dioxide behaves in a particular way inside a deep lake. It dissolves into the cold bottom water, the way bubbles dissolve into a sealed bottle of fizzy drink, and the great pressure of the water above holds it there. In Lake Nyos, the water lies in layers that almost never mix. The surface layer stays at the surface, and the deep layer stays at the bottom, growing richer in gas year after year, decade after decade. There was no smell, no colour and no warning at the surface. The lake simply kept its secret, under pressure, for a very long time.
About nine o’clock, a rumble
At about nine o’clock that night, people who lived to tell the story later remembered a low rumbling sound coming from the direction of the lake. Some described a noise like distant thunder or falling rock. Then, in the darkness, the deep water of Lake Nyos finally gave up its gas all at once.
Scientists who studied the lake afterwards believe that around 1.2 cubic kilometres of gas escaped in that single release. A column of water and foam surged up from the surface, and a wave at least 25 metres high swept across one shore, flattening trees. The gas cloud rose high at first, but carbon dioxide is heavier than air, so it sank back down and poured over the land like a slow, invisible flood. In places the cloud was about 50 metres thick. It travelled down the valleys at speeds estimated between 20 and 50 kilometres per hour, and it remained deadly for roughly 25 kilometres from the lake.
Nobody could see it coming. Nobody could smell it, because carbon dioxide itself has no odour. It simply pushed the breathable air out of the low places, and the people and animals sleeping there lost consciousness without ever understanding that anything was wrong.
Waking up in a silent valley
Some survivors did wake up, hours later, confused and weak. Several of them remembered a strange smell, like rotten eggs or gunpowder. This detail puzzled investigators for a long time, because when scientists tested the lake water, carbon dioxide was the only gas they found in it. Medical researchers who studied the disaster later suggested an explanation: at lower, non-fatal levels, carbon dioxide can play tricks on human senses, and some people exposed to it report smelling sulphur-like odours that are not really there.
One survivor, Joseph Nkwain of Subum, described how he woke unable to speak, smelling something terrible, and heard his daughter making strange snoring sounds in her sleep. When he tried to reach her, he collapsed. He slept again until the following afternoon, and when he finally managed to reach his neighbours’ houses, he found that they had all died. Riding his motorcycle through Nyos afterwards, he saw no sign of any living thing at all.
By the time help arrived, 1,746 people had died, along with around 3,500 livestock and great numbers of birds and smaller animals. About 4,000 people fled the area. The lake itself had changed colour: iron-rich water from the depths had risen to the surface and rusted in contact with the air, turning the blue water a deep, dull red. The water level had dropped by about a metre.
The warning that had come two years before
The hardest part of the story is that nature had already given a warning, and nobody had been able to read it. Two years earlier, on 15 August 1984, nearby Lake Monoun, roughly 100 kilometres away, had released a smaller cloud of the same gas in the middle of the night. Thirty-seven people died beside Monoun, and at the time the cause was a complete mystery.
Only after Nyos did scientists understand that the two events were the same rare phenomenon. An occurrence of this kind is called a limnic eruption, and in all of recorded history, Monoun and Nyos are the only two that are known to have happened. The warning of 1984 had been real. The science to interpret it simply did not exist yet.
The search for an explanation
Investigators from Cameroon and from other countries arrived to face a silent valley and a question that sounded impossible: how does a lake kill?
At first, many explanations were considered. Some thought a hidden volcanic eruption had poisoned the air. Others wondered about toxic gases of a different kind. But the evidence kept pointing back at the lake itself. The pattern of the deaths followed the low ground exactly, the way water follows a valley. The lake water still held enormous amounts of dissolved carbon dioxide. The red surface told its own story about deep water rising violently to the top. Step by step, the teams concluded that the lake had overturned and released its stored gas in one great breath.
One question has never been fully answered. What disturbed the water that particular night? Most geologists suspect a landslide into the lake. Others have suggested a small volcanic event on the lake bed, or cool rainwater upsetting the balance of the layers. Some scientists believe the gas had simply built up so close to its limit that no outside trigger was needed at all. To this day, the honest answer is that nobody knows for certain, and a careful reader should hold that uncertainty in mind.
Pipes that let the lake breathe out, slowly
Understanding the disaster brought a frightening thought with it: the gas was still down there, and it was still building. Unless something was done, Nyos could one day do the same thing again.
The solution that engineers chose is beautifully simple. A pipe is lowered from a raft into the deep, gas-rich water. A pump is used only to start the flow. As the deep water rises inside the pipe, the pressure on it falls, the dissolved gas begins to fizz, and the bubbles themselves push the water upward in a frothy fountain. From that moment, the pipe keeps working on its own, powered by the very gas it is removing. It is, in effect, a tiny, controlled version of the eruption, running gently day and night so that the real one never has to happen.
The first permanent pipe was installed at Lake Nyos in 2001, and two more were added in 2011. Lake Monoun received degassing pipes as well, and carbon dioxide detectors and warning systems were placed near both lakes. In 2019, researchers reported that the degassing at Nyos had settled into a steady state, and that even a single pipe could keep the gas at a safe level far into the future, without any external power. Scientists also watch other deep volcanic lakes, including Lake Kivu on the border of Rwanda and the Democratic Republic of Congo, which is far larger than Nyos. Studies there continue, and a study released in 2020 suggested that the risk at Kivu does not appear to be growing, though the watching continues.
I find something quietly hopeful in the patience of that arrangement: no wall, no weapon, just a pipe, a raft, and people who return year after year to measure the water and make sure a sleeping valley stays safe.
Questions people ask
What is a limnic eruption, in simple words? It is a sudden release of gas from a deep lake. Carbon dioxide builds up in the bottom water over many years, held down by pressure. If the water is disturbed, the gas escapes all at once, like the fizz rushing out when a shaken fizzy drink is opened, except on the scale of an entire lake.
Why did the victims not run away? Almost everything about the event worked against them. It happened at night, while people slept. The gas could not be seen, and carbon dioxide has no smell of its own. It moved faster than a person can run for any distance, and it caused unconsciousness very quickly.
Could Lake Nyos do it again? The gas is still entering the lake, which is exactly why the degassing pipes matter. The pipes steadily remove the gas, and scientists reported in 2019 that the levels were being held safely. The lake is monitored, and the pipes are considered the reason the valley below is far safer than it was in 1986.
Are there other dangerous lakes in the world? Lake Monoun, also in Cameroon, is the only other lake known to have erupted in this way. Lake Kivu in Central Africa holds large amounts of dissolved gas and is watched carefully by scientists, although later research suggests its risk is not increasing. Many other crater lakes in Cameroon have also been checked.
Why did the lake turn red? The deep water of Lake Nyos contains dissolved iron. When the eruption dragged that water to the surface, the iron met the air and rusted, in the same way that iron rusts on land. For a while, the lake wore the colour of that terrible night, before slowly returning to blue.
