"The Lake Nyos disaster took everybody by surprise. We had no warning signals before it ever took place," said Isaac Konfor Njilah, a Cameroonian geologist and head of a laboratory in the department of Earth Sciences at the University of Yaounde.
On the night of August 21, 1986, Lake Nyos suddenly released a massive cloud of carbon dioxide that spread through nearby valleys and villages. The gas replaced oxygen, suffocating people and animals as they slept.
Located in Cameroon's Northwest Region, about 315 kilometers (196 miles) northwest of Yaounde, Lake Nyos sits inside an ancient volcanic crater in the Oku volcanic field. At the time of the incident, the area's population exceeded 3,000, with many families relying on farming and cattle ranching.
The disaster claimed the lives of about 1,700 people and some 3,500 head of livestock, making it one of the deadliest natural disasters in Cameroon's history.
How did a lake become deadly?
Following the disaster, scientists from Cameroon and abroad launched extensive investigations to determine what had happened.
They concluded that carbon dioxide from deep underground had been accumulating at the bottom of the lake for years, trapped beneath more than 200 meters (656 feet) of water.
Before the disaster, the upper layers of the lake contained oxygen-rich water, while the deeper layers were saturated with dissolved carbon dioxide. Njilah explains that water has an insatiable appetite for carbon dioxide.
"So when carbon dioxide comes from the [Earth's] mantle, it is being dissolved continuously in the lake," Njilah told DW.
Scientists often compare Lake Nyos to a giant bottle of carbonated water. For decades, carbon dioxide built up in the lake's depths. When that balance was disturbed, the gas escaped in a rapid chain reaction, releasing a deadly cloud that rolled through nearby communities.
Researchers believe a landslide may have triggered the release by disturbing the gas-rich waters at the bottom of the lake.
Carbon dioxide is harmless at normal atmospheric levels. But in high concentrations, it acts as an asphyxiant, displacing oxygen in the air. Because the gas is heavier than air, it flowed downhill into villages, where many residents died in their sleep before realizing anything was wrong.
Tragedy at another lake in Cameroon just two years prior
The Nyos tragedy was not a first in Cameroon. Two years earlier, on August 15, 1984, a similar event occurred at nearby Lake Monoun, about 100 kilometers southeast of Nyos. A sudden release of carbon dioxide killed 37 people.
Scientists were still investigating the Monoun disaster when the far deadlier catastrophe struck at Lake Nyos. Researchers eventually concluded that the deaths were caused by a rare phenomenon known as a limnic eruption, in which gas-rich deep waters suddenly rise to the surface and release dissolved gases.
In the years that followed, engineers installed degassing pipes in Lake Nyos to gradually release carbon dioxide trapped deep below the surface.
Today, Njilah says the lake no longer contains enough dissolved carbon dioxide to trigger a disaster.
"The lake is very safe for the population to return," he told DW.
Survivors still live with the trauma
For survivors, however, the fear remains. Tsang Emelda Njah was just 5 years old when the disaster happened.
"We were at home, and there was a big vibration of the house," she recalled. "We thought it was just the house that was about to collapse. But we were not sure what it was. So we ran out."
Kum Samuel, who was 17 at the time, returned to his village and was confronted with a scene of pure horror.
"I saw my parents lying there dead. Chickens, flies and birds. Dogs together with our elders. I wasn't sure what to do."
For Ndong Frederick Bah, the losses were deeply personal.
"Nine of us went to bed that night, and I was the lone survivor of the family," he told DW, adding that the next day, people who had heard about the disaster came and rescued him.
Entire communities were later relocated to new settlements. Although scientists have reduced the risk of another gas release, many survivors still struggle to overcome the trauma.
"I am a child of Nyos," said Tsang Emelda Njah. "But I am still afraid to go close to the lake. That's because whatever killed people there, I am not sure whether or not it has been stopped from ever happening again. That is why we are still afraid."
Anglophone crisis harms restoration
While scientists consider the lake safe, long-term reconstruction efforts have been slowed by Cameroon's Anglophone crisis.
The conflict erupted in 2016 between government forces and separatist groups in the country's primarily English-speaking Northwest and Southwest regions, where Lake Nyos is located.
According to Njilah, plans to rehabilitate affected communities and encourage people to return to their ancestral lands have been disrupted by insecurity, with the area now difficult to access.
"It is the plight of the survivors to go back home. They want to go back to their ancestral land so that they are able to pour libations on the graves of their forefathers," he said.
The ongoing violence has also hindered scientific work and development projects.
"Nobody can venture [there,] unless you want to have a bullet lodged in your head," Njilah said. "It has affected government and international agencies who are working there."
