The evening of August 21, 1986, was like any other night around Lake Nyos, a deep crater lake high on the slopes of the Massif du Mbam mountain range in northwest Cameroon. Some residents of the local villages huddled around their cooking fires, tucking into a late dinner. Many others, tired from a busy day at the market, were already asleep in their grass-roofed huts.

Around 9:30 p.m., those who remained awake heard a strange rumbling noise coming from the direction of Lake Nyos. Within minutes, nearly 1800 people would be dead.

That night, Lake Nyos expelled a jet of water over 300 feet high, releasing with it years' worth of carbon dioxide that had collected in the lake. A cloud of gas rose into the sky before descending over the hilltops and heading toward the unsuspecting villagers. At 160 feet thick and traveling between 12 and 31 miles per hour, there was little chance of escaping it. As it swept past the huts, the hot cloud of carbon dioxide displaced the air, suffocating almost everyone it came into contact with until it eventually dissipated.

Lake Nyos was known by locals as the “good” lake for its clean drinking water. But on that night in 1986, it was responsible for one of the deadliest natural disasters in African history.

A Disastrous Disturbance

Water shooting from an artificial carbon dioxide vent in Lake Nyos in 2006.Bill Evans/USGS, Wikimedia Commons // Public Domain

Nios, the village closest to the lake, was the worst affected. A man who traveled to Nios on his motorbike the following day discovered it strewn with the bodies of people and animals. He couldn’t find a single person alive.

The man rushed back to his village, Wum, around five miles away. The first survivors had just started to arrive. They later recalled choking on the air before passing out. Some remained unconscious for two days, only to wake up and find that their entire family had died.

As word spread about the disaster, scientists flocked to Cameroon to try to understand what had happened. Tests of the water soon revealed the lake had unusually high levels of carbon dioxide. The carbon dioxide levels were so high that when the scientists tried to pull the water samples to the surface, pressure from the gas caused the containers to burst. They theorized that carbon dioxide had accumulated at the bottom of Lake Nyos until something had disturbed it. That disturbance caused a chain reaction that forced the gas out from the bottom of the lake and into the atmosphere in a rare natural phenomenon called a limnic eruption.

In the following months, U.S. research chemists found that the carbon dioxide levels in Lake Nyos were increasing at an alarming rate. Something had to be done to keep another disaster at bay.

Geologists from the Cameroon Ministry of Mines, Water, and Power proposed installing a pipe system into the lake designed to allow the controlled release of carbon dioxide from its bed up through the surface. Starting with small pipes the diameter of a garden hose, scientists began to test the idea in 1990, swapping them out for progressively larger pipes in the following years. Meanwhile, all villagers within an 18-mile radius of the lake were evacuated. Their villages were destroyed to prevent them from going back.

Though the pipes provided a temporary solution, there were still 5500 tons of carbon dioxide collecting in Lake Nyos every year from the magma chamber deep below the volcanic line the crater lake sits atop. Funding was eventually secured to install the first permanent pipe in 2001, followed by an additional two pipes in 2011. It took another five years for the carbon dioxide to reach safe enough levels for villagers to return and rebuild their communities—three decades after the disaster that had claimed so many of their friends and family.

The Threat of Lethal Lakes

Lake Kivu is an enormous reservoir of gas.Steve Evans, Wikimedia Commons // CC BY 2.0

What triggered Lake Nyos's limnic eruption remains unknown. It could have been something as small as a rock falling into the water, or even a strong gust of wind. Once the scientists began figuring out what could have caused the disaster, they began to look for instances of similar eruptions [PDF]. It didn’t take them long to find one.

Just two years earlier in Lake Monoun, 59 miles away from Lake Nyos, nearby villagers heard a loud boom. In the following hours, 37 people mysteriously died. Until then, the strange occurrence hadn't garnered much attention. But in light of the Lake Nyos disaster, it was proof that the problem was greater than anticipated.

Scientists now believe that only three lakes in the world accumulate such deadly levels of carbon dioxide in their depths—Nyos, Monoun, and Lake Kivu on the border of Congo and Rwanda. While Lake Nyos and Lake Monoun have both been declared safe, the same cannot be said for Lake Kivu. Around 2 million people live in the valleys surrounding the lake, which is 1700 times bigger than Lake Nyos and twice as deep. Though Rwanda has begun using methane from Lake Kivu as a source of energy, large scale efforts to completely de-gas the lake have yet to be made. Until that happens, history threatens to repeat itself as danger bubbles silently beneath the surface.