What came down from Lake Nyos and killed 1,746 villagers?

August 1984

Thirty-seven people died near Lake Monoun, in Cameroon. The bodies were found beside a road and did not have the injuries an explosion would have left.

One hypothesis pointed to carbon dioxide built up in the lake. It was hard to accept that a lake could let go of enough gas to kill beyond its own shores, and no explanation stuck.

21 August 1986, at night

The families of Nyos, Cha, Subum and Fang went to bed around Lake Nyos. A few hours later, whole villages had stopped answering.

The houses were still standing. There was no fire, no flood, no new crater across the paths.

Joseph Nkwain was with his family. He said afterwards that he heard his daughter breathing badly, that he tried to reach her bed and that he lost consciousness before he got there.

Outside, the thing was moving along the ground. It went into the low houses, followed the slopes and came down through the valleys. Almost everyone who died was asleep.

22 August 1986

The first people into the villages were neighbours from the settlements round about, who came to collect and bury bodies. The formal response teams set out for the area the following day.

Those who survived had woken hours later, dizzy and without strength. The ones sleeping higher up had better odds, and people standing on a hill when the current passed below them survived too.

They found motionless cattle, neighbours lying where they fell and dead flies. That last detail was the strangest: whatever it was had reached the insects and had not broken a single door.

Several survivors spoke of a smell of rotten eggs or gunpowder. Carbon dioxide has no smell. Where that smell came from was argued over for years: the 1987 report suggested a perception brought on by extremely high concentrations of CO2, and later work rejected that explanation. It is still open.

The count ended at some 1,746 people and around 3,500 head of livestock, spread across the four villages. They were not all the same distance from the water: whatever it was had come out of the basin and travelled along the shape of the ground.

The first hypotheses

A volcanic eruption looked like the immediate answer: the lake sits in a crater and there is magmatic activity underneath.

But no fresh ash turned up, the bodies had no burns, the houses had not fallen and the vegetation showed none of the heat damage an eruption leaves. What was intact was everything: the kitchens, the beds, the fences.

The idea of a weapon went round as well. That didn't fit either: no explosive residue, no impacts, no blast zone to reconstruct an attack from.

They had to look at the least spectacular setting. The lake was still there.

The lake

The water had changed colour. It used to look blue; now it had a rusty red tone. That meant deep water, loaded with iron, had come up to the surface and oxidised on contact with the air.

Near the shore, several trees lay flat in the same direction. Something had shifted water with a great deal of force.

Cameroonian, French and American teams began to study the water layer by layer, lowering instruments towards the bottom instead of making do with the surface. The difference between top and bottom was the key: the deeper you went, the more dissolved gas there was.

Measuring it was not straightforward. Bring a sample of deep water up to the surface and the bottle itself becomes the experiment: the pressure drops on the way and the gas starts coming out before it reaches the top. The instrument had to go down to the gas.

The lake is around 200 metres deep. Below it, magma releases carbon dioxide, which seeps into the deep water without needing any eruption of lava.

The mechanism

At the bottom, the pressure of the whole water column keeps that gas dissolved. The lake is a 200-metre bottle of fizzy drink: while the pressure holds, the gas stays dissolved in the liquid. When the pressure drops, bubbles come out.

At Nyos, the deep water had been taking in CO2 for a very long time. The lake did not need to boil, nor the volcano to explode that night. It was enough for some of that saturated water to start rising.

The exact trigger was never established: it could have been a landslide, a cooling of the surface, heavy rain, or a mixture of things.

What came next could be reconstructed. As it rose, the water was under less pressure and let go of bubbles. The bubbles made the mixture lighter, so it rose faster. That movement dragged up more loaded water from the bottom, which lost pressure, released more bubbles and speeded up again.

The reaction fed itself.

In a few hours an enormous quantity of carbon dioxide came out of the lake. The phenomenon has a name: limnic eruption. It throws out no lava; it empties in one go the gas that has built up in the deep layers.

The route

CO2 is denser than air. Once over the crater rim, the cloud neither rose nor dispersed: it fell towards the low ground, went into the valleys and moved along close to the surface at an estimated 20 to 50 kilometres an hour.

Where it gathered, it pushed out the breathable air at the height of a body lying down. A sleeping person could lose consciousness without ever getting up. Anyone higher up was outside the most concentrated part.

Animals graze and sleep at ground level. Hence the 3,500 head of livestock, and the flies.

It also explains why some people lived and the person beside them did not. In the same house, the person sleeping in the loft lived and the person sleeping downstairs died. Some people stayed on their feet and lost consciousness when they bent down. And those who set off uphill, towards the ridges, were out of the cloud within minutes; those who ran along the path, which follows the valley floor, ran inside it.

The other problem

Studying the basin turned up a danger that had nothing to do with the gas. The lake water is held in by a natural wall of loosely packed volcanic rock, and that wall was eroding. If it had given way, millions of cubic metres would have emptied down the valley in one go, towards the villages on the route to Nigeria. That risk was tackled separately: around 2013 cement was injected into the rock and a concrete slab laid on top to stop the erosion. Today it is considered averted.

The years that followed

Monoun stopped looking like an anomaly: two Cameroonian crater lakes had built up CO2 at depth and let it go all at once. The question became how much gas was left inside Nyos and whether it could load up again.

Taking it all out at once could destabilise the water; leaving it alone kept the danger. It had to be released little by little.

Michel Halbwachs, a physicist and engineer at the University of Savoy, and Jean Grangeon showed how to do it with a vertical pipe.

2001

The first degassing pipe started working at Lake Nyos. It went down to the deep water loaded with CO2. Once pumping began, that water rose; on the way it lost pressure and the gas formed bubbles; the mixture got lighter and gained speed until it came out of the top as a fountain.

And once started, it kept going by itself: the escaping gas held the column up, with no need to pump from the bottom.

2011

Two more pipes were installed. The magma is still down there supplying carbon dioxide; the pipes only stop it building back up to the level it had reached in 1986.

The survivors from the four villages were evacuated to camps and many took years to be able to return. The valley repopulated slowly, and not always with the same people.

The testimony

Joseph Nkwain survived. From his account of that night, this sentence remains: "I could not speak."