A small lake in Africa killed 1,700 villagers and 3,500 livestock overnight when it suddenly released 300,000 tons of carbon dioxide, suffocating everything within 16 miles. Scientists still don’t know what triggered the event

This is the story of the Lake Nyos disaster that took place on 21 August 1986, a limnic eruption in northwestern Cameroon.

The eruption triggered the sudden release of about 100,000–300,000 tons (1.6m tons, according to some sources) of carbon dioxide (CO2). The gas cloud initially rose at nearly 100 kilometers per hour (62 mph) and then, being heavier than air, descended onto nearby villages, displacing all the air and suffocating people and livestock within 25 kilometers (16 mi) of the lake.

A degassing system has since been installed at the lake, with the aim of reducing the concentration of CO2 in deep waters and therefore the risk of further eruptions.

It is not known what triggered the catastrophic outgassing. Most geologists suspect a landslide, but some believe that a small volcanic eruption may have occurred on the bed of the lake. A third possibility is that cool rainwater falling on one side of the lake triggered the overturn. Others still believe there was a small earthquake, but as witnesses did not report feeling any tremors on the morning of the disaster, this hypothesis is unlikely. Whatever the cause, the event resulted in the supersaturated deep water rapidly mixing with the upper layers of the lake, where the reduced pressure allowed the stored CO2 to effervesce out of solution.

It is believed that about 1.2 cubic kilometers (0.29 cu mi) of gas was released. The normally blue waters of the lake turned a deep red after the outgassing, due to iron-rich water from the deep rising to the surface and being oxidized by the air. The level of the lake dropped by about a meter and trees near the lake were knocked down.

Scientists concluded from evidence that a 100 m (330 ft.) column of water and foam formed at the surface of the lake, spawning a wave of at least 25 meters (82 ft.) that swept the shore on one side.

Carbon dioxide, being about 1.5 times as dense as air, caused the cloud to “hug” the ground and move down the valleys, where there were various villages. The mass was about 50 meters (160 ft.) thick, and travelled downward at 20–50 kilometers per hour (12–31 mph). For roughly 23 kilometers (14 mi), the gas cloud was concentrated enough to suffocate many people in their sleep in the villages of Nyos, Kam, Cha, and Subum. About 4,000 inhabitants fled the area, and many of these developed respiratory problems, lesions, and paralysis as a result of the gas cloud.

Following the Lake Nyos disaster, scientists investigated other African lakes to see if a similar phenomenon could happen elsewhere. Lake Kivu in the Democratic Republic of Congo, 2,000 times larger than Lake Nyos, was also found to be supersaturated, and geologists found evidence that outgassing events around the lake happened about every thousand years.