In August 1986, approximately 1800 people living in a 15-mile radius of Lake Nyos in Cameroon didn’t live to see the morning, dying by suffocation from toxic gas exhaled from the bottom of the lake. Some survivors awoke hours after falling unconscious. They reported they heard a strange and loud noise but when they awoke, they were in complete silence, without even insects buzzing around. People and livestock lay scattered, dead. The small lake (1.5 sq km) had changed color to reddish and murky. Later, chemical, isotopic, geologic, and medical evidence led to the conclusion that all died from carbon dioxide asphyxiation.

Lake Nyos after the eruption. Photo US Geological Survey.

Overturning of the lake

This is classified as a limnic eruption (‘limnic’ referring to fresh water/lakes). It is also called a “lake overturn”. Many lakes or ponds experience a seasonal turnover when the water from the bottom rises and the surface water sinks. But, a gas-propelled turnover is violent. Water is displaced by the explosively rising gas column and can cause a tsunami on shore.

These disasters are rare for several reasons. First, there must be a large amount of carbon dioxide (or methane) that becomes trapped under pressure. While this can occur from vegetation decomposition, a more voluminous source of gas is from volcanic degassing. In some volcanic areas around the world, deadly events can occur when vented gases become trapped in surface depressions or caves into which people or animals wander. But for a lake to become saturated with gas, it must be deep (Nyos is over 600 ft deep) and cold at the bottom. There is no seasonal mixing in these lakes because of the tropical location.

Graphic by Theresa Kayzar from

Limnic eruptions have been observed twice in historical times including Nyos and a few years earlier at a nearby location (see below). Certainly, they have happened in prehistory and can happen again in lakes in this equatorial region overlying volcanic vents. Though the lakes are over on active volcanoes, gases from the magma chambers that remain are released into the lower lake level (hypolimnion) where the gas dissolves into the water. The situation becomes analogous to a carbonated bottle of soda. Under pressure, the gasses stay dissolved until the pressure is lessened and the gas bubbles out, surging to the surface.

The lake released an eruption of gas similar to what happens when pressure is released from a can of carbonated liquid. In this case, the candy pieces represent rocks that disturbed the unstable layers, causing an eruption.

When the lake is supersaturated with gas, the layers are very unstable. A small trigger is all that is needed to spark the runaway process of explosive degassing. This trigger can be a rockslide, a small release of magma, an earthquake, a temperature change such as warming of the lower layer, or other surface disruption (even strong winds).

Various dangerous gases are released from volcanic areas including carbon dioxide, hydrogen sulfide, and methane. For most volcanic lakes, the degassing takes place gradually without great risk of a catastrophe. At Lake Nyos, and a few years earlier (1984) at Lake Monoun nearby, the sudden release of massive amounts of carbon dioxide from the lake bottom displaced oxygen along the shore villages suffocating people and animals. According to a fascinating report in Smithsonian magazine, observers of the Monoun event thought the town was targeted by a militant chemical attack or the government itself.

Volcanologist Harold Sigurdsson went to Lake Monoun immediately after the event to attempt to discern what had happened, but he found no evidence of an eruption. Here is what happened next:

Sigurdsson, after a few weeks, began to conclude that carbon dioxide from magma degassing deep under Lake Monoun had percolated up into the lake’s bottom layers of water for years or centuries, creating a giant, hidden time bomb. The pent-up gas dissolved in the water, he believed, suddenly had exploded, releasing a wave of concentrated carbon dioxide. He wrote up his findings, calling the phenomenon “a hitherto unknown natural hazard” that could wipe out entire towns, and in 1986, a few months before the Nyos disaster, he submitted his study to Science, the prestigious U.S. journal. Science rejected the paper as far-fetched, and the theory remained unknown except to a few specialists.Then Lake Nyos blew up, killing 50 times more people than at Monoun.

Triggering mechanisms for the 1984 and 1986 limnic eruptions are still uncertain. A limnologist who had sampled (the surface) of Lake Nyos a year prior to the eruption also missed the flagging any anomalies when he only sampled near-surface waters which were normal. Upon hearing of the second catastrophe, some scientists suspected the same cause was to blame. When that same researcher returned to the lake, it was now rust-colored which suggested iron-rich water from the oxygen-poor bottom had oxidized at the surface after the CO2 dissipated on the wind.

Lake Nyos. NASA Landsat 8 – Public Domain, Wikimedia Commons.

Maleficent waters

The local people had told stories of disaster coming from the lakes. Lakes are said to be the abode of spirits of ancestors and may bring death. Anthropologist Eugenia Shanklin studied the folklore of the area and suggested that stories about the waters “misbehaving” may be linked to events in the geologic past. She noted that some cultures in the area require houses near lakes be situated on high ground. 
Of course, this would make sense for a number of reasons, and the link between “maleficent” lakes and the limnic eruption may be spurious. Still, the locals conduct rituals to appease the spirits and ask for protection against their future wrath.

The aftermath near Lake Nyos.

This type of phenomenon is periodic. Nyos continues to receive an influx of gas (5,500 tons of CO² a year) which means another turnover would inevitably occur and threaten the people who return to their land. In 2001 a low-tech degassing pipe was installed (funded by the U.S. Office of Foreign Disaster Assistance at $433,000) to allow for the slow and harmless dispersion of a small amount of gas. Two more pipes were added in 2011 to decrease the gas levels even more rapidly, and alarms were installed to warn of dangerous levels of gas accumulation.  Over 30 years later the lake shore is still poisoned from the eruption. The government will not support resettlement of the area until they are certain a similar disaster can be averted, though some people are filtering back.

Lake Nyos

Lake Kivu, nearer to the active volcanic complex of Nyiragongo and Nyamulagira, is a much larger lake at risk of limnic eruption. The vast concentrations of methane and CO² at this lake much larger than at Nyos and Monoun. But the dissolved methane is being extracted from the lake via piping on a small scale. Economically, the resource would be a huge benefit to the area. However, the risk of a similar catastrophic explosion remains a serious concern.

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