On August 21, 1986, a strange rumbling noise was heard at Lake Nyos in Northwest Cameroon. The following morning, 1,746 people and over 3,500 livestock within 25 kilometers (16 miles) of the lake were found dead.
Ephriam Che, a farmer who lived in a nearby mud-brick house on a cliff, heard the rumbling at around 9 pm. Thinking nothing of it, he went to bed shortly afterward. When he awoke the following day, he headed down to a waterfall and found it strangely dry. Weirder still, the whole place was eerily quiet, with no sounds from birds or animals or even insects to be heard. Terrified, he continued downhill towards the village by the lake, when he heard shrieking.
Halima Suley, a cowherd who lived in the village, was standing there, desperately calling for Che's attention. Around her were the dead bodies of 31 members of her family, and 400 dead cattle.
"There were no flies on the dead," Che told The Smithsonian, because the flies were dead as well.
Others had similar stories. "I was sitting, just sitting among the dead people inside the house, some of them were outside, some of them behind the houses and it was animals everywhere lying, cows, dogs, cows, everything, so I was confused by then," Monica Lom Ngong told the BBC. "All the family, we were 56, but 53 died."
There was no sign of any struggle here or in the village below, where hundreds of others lay dead – roughly where they would have been at around 9 pm when that ominous sound had occurred. The only other clues were the smell of rotten eggs, and strange marks on the bodies of the dead and the living.
"When I wake up I had burns on my left arm," Monica said. "By then I was feeling no pains. The arm was giving in a way that it was nearly having rotten because of the wounds."
It wasn't clear to anybody what had happened, with people putting it down to chemical attacks as well as more superstitious explanations. Those that had survived – mostly people who quickly sought higher ground away from the lake – described family members napping on the floor dying where they lay, while they were spared, as well as a cloud of gas rolling out from the lake at astonishing speed. Some of the people turned out to be merely unconscious and woke the following day to see the scenes of carnage around them.
Scientists descended upon Nyos to investigate in the coming weeks, finding bodies strewn everywhere, and a lake that had turned from blue to red. Of those who had survived, many were suffering from vomiting, diarrhea, and hallucinations, all of which were symptoms of CO2 poisoning.
Survivors talk about the disaster.
Samples from the lake confirmed the cause of the incident. Lake Nyos was formed in a volcanic crater, which was producing CO2. Usually, in volcanic lakes, the gas would be released as the water is disturbed and sloshes around. Lake Nyos, however, was unusually still – and as a result, there had been a buildup of CO2 in the sediment for decades, unknown to anyone nearby. It's not known what exactly disturbed the lake that day – whether it was a landslide or just sudden release of the gas – but when it went off, about 1.2 cubic kilometers (0.24 cubic miles) of CO2 was released in about 20 seconds, spilling over and asphyxiating anyone in its path. The gas is denser than the surrounding air, so it spared those who were standing up or happened to be on higher ground, while anyone sleeping on the floor would never wake up again.
It's not the only lake to have this problem. Lake Kivu, between the Democratic Republic of the Congo and Rwanda, is much, much bigger than Nyos and surrounded by a far denser population. Studies have shown extinction events in the lake take place every thousand years or so, so it's likely not a matter of if another extinction event will happen, but when.