A limnic eruption is a rare natural disaster in which a massive burst of carbon dioxide (CO2) suddenly escapes from the deep water of a lake, forming an invisible cloud of gas that can suffocate people and animals for miles around. Only two confirmed limnic eruptions have occurred in recorded history, both in Cameroon in the 1980s, but together they killed nearly 1,800 people. The phenomenon is sometimes called a “lake overturn” because it involves deep, gas-saturated water rising and releasing its dissolved gas all at once.
How Gas Builds Up in a Lake
Limnic eruptions can only happen in a specific type of lake: one that sits in or near a volcanic area and has water layers that don’t mix. In most ordinary lakes, seasonal temperature changes cause the water to circulate top to bottom, which prevents any gas from accumulating. But some volcanic lakes develop a permanent separation between their upper and lower layers, held in place by differences in temperature, salinity, and dissolved gas content. Scientists call this type of lake “meromictic.”
In these lakes, CO2 of volcanic origin seeps up through the lake bottom and dissolves into the cold, high-pressure deep water. Dissolved CO2 actually increases water density, which reinforces the layering and keeps the gas-rich water trapped at depth. Over years or decades, the concentration of dissolved gas climbs steadily. The deep water essentially becomes carbonated, like a sealed bottle of sparkling water held under enormous pressure. As long as the layers remain stable, the gas stays dissolved and harmless.
What Triggers the Eruption
The eruption happens when something disrupts that stability. Scientists have identified several potential triggers: volcanic activity that heats the deep water or sends a fresh surge of gas through the lake floor, a landslide that plunges into the lake and physically displaces the layers, an earthquake, or even a period of unusually cold rain that chills the surface water enough to make it denser than the layers below. At Lake Averno in Italy, for example, researchers found that when surface temperatures drop below about 10°C, the upper water becomes denser than the water beneath it, making the layering unstable and prone to overturning.
Once the stratification breaks down, deep water begins rising. As it moves upward, the pressure on it drops, and dissolved CO2 starts forming bubbles, just like opening a soda bottle. Those bubbles make the rising water even more buoyant, which pulls more deep water upward, which releases more gas. This self-reinforcing chain reaction can send an enormous volume of CO2 bursting from the lake surface in minutes. At Lake Monoun, researchers believe the eruption was powerful enough to displace lake water onto the shore, eroding the surrounding landscape.
The 1984 and 1986 Cameroon Disasters
The first recognized limnic eruption struck Lake Monoun in Cameroon on August 15, 1984, killing people in nearby communities. At the time, the cause was poorly understood. Subsequent research confirmed that magmatic CO2 dissolved in the lake water had been explosively released from the surface, forming a low-oxygen air mass that spread outward and suffocated victims.
Two years later, on August 21, 1986, a far larger eruption occurred at Lake Nyos, another volcanic crater lake roughly 100 kilometers away. Around 1,700 people died. The CO2 cloud, heavier than air, flowed downhill through valleys and settled over villages. Interviews with survivors and pathological studies confirmed that victims lost consciousness rapidly and died from CO2 asphyxiation. Some survivors reported smelling gunpowder or rotten eggs, hinting that trace amounts of sulfur gases were present, but CO2 was the only gas detected in lake water samples and was clearly the primary killer.
One survivor, Joseph Nkwain from the village of Subum, described waking hours later unable to speak, covered in wounds he couldn’t explain, with a strange starchy residue on his body. His daughter had been snoring in a “terrible, very abnormal” way. Skin lesions found on survivors turned out to be pressure sores from lying unconscious for extended periods, not chemical burns. There was no evidence of hot gases or chemical exposure beyond the CO2 itself.
Why the Gas Is So Deadly
CO2 is colorless and, at the concentrations involved, essentially odorless. Because it is about 1.5 times denser than normal air, the released gas hugs the ground and pools in valleys and low-lying areas. During a limnic eruption, the gas mixes with ambient air to create a mass with dangerously low oxygen levels. People caught in this cloud suffocate without ever realizing what is happening. Victims in the Cameroon events lost consciousness within moments, many dying in their sleep. Livestock, wildlife, and even insects over a wide area were killed.
Lake Kivu and Ongoing Risk
The lake that worries scientists most today is Lake Kivu, which straddles the border of the Democratic Republic of Congo and Rwanda. It dwarfs the Cameroonian lakes: roughly 3,000 times larger than Lake Nyos, with two to four orders of magnitude more dissolved CO2. It also contains substantial quantities of dissolved methane. About two million people live along its shores.
There is no standard method for precisely measuring the gas concentrations in Lake Kivu, which complicates risk assessment. The scale of a potential eruption there could be catastrophically larger than anything seen in Cameroon. The lake sits in the East African Rift, one of the most volcanically and seismically active regions on Earth, meaning external triggers like earthquakes and volcanic activity are a persistent concern.
How Scientists Are Reducing the Danger
After the Lake Nyos disaster, engineers developed a straightforward but effective solution: vertical pipes that bring gas-rich water from the lake bottom to the surface, where the CO2 escapes harmlessly into the atmosphere in low concentrations. Once flow through a pipe is mechanically started, the buoyancy of the rising bubbly water makes the process self-sustaining, requiring no external energy.
Controlled degassing began at Lake Nyos in 2001 and Lake Monoun in 2003. Early critics worried that the pipes themselves might destabilize the lakes and trigger another eruption. Measurements have shown the opposite. The overall structure of the water layers has not been compromised, and local stability has actually increased in the zones around the pipes. Gas content in both lakes was reduced by 12 to 14 percent in the initial years of operation.
The progress, however, is slow. With a single pipe, the gas concentration at the pipe’s intake gradually drops, which slows the removal rate. Drawing Lake Nyos down to safe levels could take decades. During that time, enough gas remains dissolved to cause lethal consequences if released. At Lake Kivu, the challenge is orders of magnitude greater, though methane extraction projects there serve a dual purpose: generating energy and reducing dissolved gas levels.
Other Lakes With Potential Risk
While the Cameroonian lakes are the only ones to have produced confirmed limnic eruptions in modern times, any deep volcanic lake with permanent stratification can theoretically accumulate dangerous amounts of CO2. Lake Averno in Italy, located in the volcanic Campi Flegrei caldera near Naples, shows this pattern clearly. Researchers have documented fish kills and surface color changes during winter overturns, caused by iron-rich deep water reaching the surface and reacting with oxygen. The lake’s deep layers are enriched with volcanic CO2, and modeling suggests that a strong enough temperature drop or a surge in volcanic gas input could push the system toward an eruption.
Researchers have begun applying paleolimnological techniques, studying lake sediment cores and shoreline erosion patterns, to identify whether other lakes around the world have experienced similar events in the geologic past. The goal is to build a global inventory of at-risk lakes before another disaster occurs, rather than after.