Lake Nyos disaster

It is not known what triggered the catastrophic outgassing.[5][6][7] Most geologists suspect a landslide, but some believe that a small volcanic eruption may have occurred on the bed of the lake.[8][9] 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 because witnesses did not report feeling any tremors on the morning of the disaster, this hypothesis is unlikely. 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.[10]

It is believed that about 1.2 cubic kilometres (0.29 cu mi) of gas was released.[11] 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 oxidised by the air. The level of the lake dropped by about a metre 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 metres (82 ft) that swept the shore on one side.[12]

Since carbon dioxide is 1.5 times the density of air, the cloud hugged the ground and moved down the valleys, where there were various villages. The mass was about 50 metres (160 ft) thick, and travelled downward at 20–50 kilometres per hour (12–31 mph). For roughly 23 kilometres (14 mi), the gas cloud was concentrated enough to suffocate many people in their sleep in the villages of Nyos, Kam, Cha, and Subum.[3] About 4,000 inhabitants fled the area, and many of these developed respiratory problems, lesions, and paralysis as a result of the gas cloud.[13]

It is a possibility that other volcanic gases were released along with the CO₂, as some survivors reported a smell of gunpowder or rotten eggs which indicates that sulfur dioxide and hydrogen sulfide were present at concentrations above their odour thresholds. However, CO₂ was the only gas detected in samples of lake water, suggesting that this was the predominant gas released and as such the main cause of the incident.[13]

Cattle suffocated by carbon dioxide from Lake Nyos

One survivor, Joseph Nkwain from Subum, described himself when he awoke after the gases had struck:

"I could not speak. I became unconscious. I could not open my mouth because then I smelled something terrible ... I heard my daughter snoring in a terrible way, very abnormal ... When crossing to my daughter's bed ... I collapsed and fell. I was there till nine o'clock in the (Friday) morning ... until a friend of mine came and knocked at my door ... I was surprised to see that my trousers were red, had some stains like honey. I saw some ... starchy mess on my body. My arms had some wounds ... I didn't really know how I got these wounds ... I opened the door ... I wanted to speak, my breath would not come out ... My daughter was already dead ... I went into my daughter's bed, thinking that she was still sleeping. I slept till it was 4.30 in the afternoon ... on Friday. (Then) I managed to go over to my neighbours' houses. They were all dead ... I decided to leave ... (because) most of my family was in Wum ... I got my motorcycle ... A friend whose father had died left with me (for) Wum ... As I rode ... through Nyos I didn't see any sign of any living thing ... (When I got to Wum), I was unable to walk, even to talk ... my body was completely weak."[3][14]

Following the eruption, many survivors were treated at the main hospital in Yaoundé, the country's capital. It was believed that many of the victims had been poisoned by a mixture of gases that included hydrogen and sulfur. Poisoning by these gases would lead to burning pains in the eyes and nose, coughing and signs of asphyxiation similar to being strangled.[7]

Interviews with survivors and pathologic studies indicated that victims rapidly lost consciousness and that death was caused by CO₂ asphyxiation. [15] At nonlethal levels, CO₂ can produce sensory hallucinations, such that many people exposed to CO₂ report the odor of sulfuric compounds when none are present. [15] Skin lesions found on survivors represent pressure sores, and in a few cases exposure to a heat source, but there is no evidence of chemical burns or of flash burns from exposure to hot gases. [15]

The scale of the disaster led to much study on how a recurrence could be prevented.[16] Several researchers proposed the installation of degassing columns from rafts in the middle of the lake.[17][18] The principle is to slowly vent the CO
2 by lifting heavily saturated water from the bottom of the lake through a pipe, initially by using a pump, but only until the release of gas inside the pipe naturally lifts the column of effervescing water, making the process self-sustaining.[19]

Starting from 1995, feasibility studies were successfully conducted, and the first permanent degassing tube was installed at Lake Nyos in 2001. Two additional pipes were installed in 2011.[20][21] In 2019 it was determined that the degassing had reached an essentially steady state and that a single one of the installed pipes would be able to self-sustain the degassing process into the future, indefinitely maintaining the CO
2 at a safe level of without any need for external power.[22]

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.

A Greek novel by Basileios Drolias focusing on the Lake Nyos disaster and the degassing of the lake was published in 2016.[23]

• Lake Monoun
• Lake Kivu
• List of natural disasters by death toll

• Google Earth view of the area around Lake Nyos