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On the night of 15 August 1984, a truck sagging with the weight of a dozen passengers trundled along a misty road in Cameroon, Africa. Although there had been no indication of a problem mere moments before, the vehicle suddenly sputtered and stalled. The driver turned the key, but the churning ignition was unable to reanimate the engine. Most of the Cameroonians clambered out of the vehicle to investigate, but two remained atop the truck. Within a few moments, each of the ten passengers who had stepped off the vehicle slumped to the ground. Within minutes, they were dead.

Those unfortunate travelers were not the only people in the area to die mysteriously that night. In the neighboring low-lying villages, twenty-seven other residents inexplicably passed away in their sleep, and an unspecified number of animals perished in the vicinity. Investigators were at a loss to explain the mass fatalities, wondering whether some exotic terrorist attack may have been to blame. Interviews with surviving villagers indicated that a distant explosion had been heard sometime after sunset, and that a light mist had appeared soon thereafter. A survey of the area found that nearby Lake Monoun had taken on a rusty tinge; but these clues were not consistent with any known weapon or natural disaster. It was not until about two years later that authorities ascertained that a mazuku, or “evil wind” had swept through the valleys of Cameroon that night, originating from the shores of the discolored lake. The water had not been tampered with by terrorists, however, nor was it the site of a volcanic eruption. The lake itself had exploded.

Although Lake Monoun resembles an ordinary lake to the naked eye, it is a rather abnormal body of water. Its basin is unusually deep and its walls are steep-sided, therefore surface winds are unable to produce enough turbulence to intermix the lake’s layers of water. For this reason, Lake Monoun’s cold lower stratum tends to remain undisturbed for decades⁠—or even centuries. This stagnant lower layer is not particularly menacing on its own, but Monoun is also located directly above a volcanic vent which slowly leaks carbon dioxide (CO2) into the lake through the basin floor. In most lakes such gas bubbles would merely rise to the top, or dissolve in the water until natural intermixing cycles brought the CO2-bearing water nearer the surface. However the high pressure and cold temperature of the undisturbed bottom layer of Lake Monoun allowed the dissolved CO2 to linger and concentrate for years.

During that deadly evening in 1984, something stirred the water at the bottom of Lake Monoun⁠—possibly seismic activity, a landslide, or rainfall. Whatever the cause, some of the supersaturated layer was nudged up from the bottom and allowed to mingle with warmer, lower-pressure waters. Although the temperature and pressure differences were only slight, the carbon dioxide crossed a critical threshold where it would no longer remain dissolved. One or more pockets of CO2 abruptly expanded into their gaseous state. These bubbles then acted as nucleation sites, causing surrounding water to give up its trapped gases as well. When these large bubbles raced towards the lake surface, their suction force drew more of the stagnant water up into the lower-pressure area, liberating even more CO2, and triggering a runaway chain-reaction.

Hundreds of thousands of tons of captive gas was belched from the depths of the lake in a matter of moments, blowing the top off the lake with tremendous force. The displaced water created a tsunami of sorts as the upper layers of the lake surged over the shores. The escaped mass⁠—made up of carbon dioxide, carbon monoxide, and traces of hydrochloric acid⁠—was heavier than the surrounding air, causing it to cling to the earth and slither down the valleys of Cameroon. The river of toxic gas was mostly invisible, with occasional patches of white mist. Almost everyone in its path was asphyxiated inside of a few minutes, although a handful fled to safety after watching their neighbors collapse, and a fortunate few were in elevated locations. Over the next few days, the lake gradually took on a red-brown tinge as the iron-rich water brought up from the deep was oxidized by the atmosphere.

The exploding-lake phenomenon, which had never been observed before then, came to be known as a limnic eruption. There are no surviving eyewitnesses of limnic eruptions, but we know that the effect is roughly analogous to an exploding soda-pop: when a sealed soda container is shaken, the turbulence causes some of the carbon dioxide in the solution to evaporate into short-lived bubbles stuck to imperfections on the inner walls of the can. These bubbles contain high pressure, but they remain small due to the internal pressure of the can. Opening the can while in this state releases that pressure, and the high pressure CO2 bubbles expand rapidly to equalize with the much lower ambient air pressure. To put it another way, a limnic eruption would look like someone threw a few billion Mentos^TM into a lake of Diet Coke^TM.

Almost exactly two years after the mysterious eruption, sometime during the night on 21 August 1986, another of Cameroon’s lakes exploded without warning. Lake Nyos, a body of water with features quite similar to Lake Monoun, let loose its own dense cloud of deadly gas. While the Monoun explosion was tragic, its death toll was dwarfed by the devastation of the second incident. Lake Nyos expelled over a million tons of gas⁠—about a cubic kilometer⁠—and the heavy cloud engulfed a populated valley. Villagers began to feel profoundly unwell, gasping for air but unable to get any oxygen. Some 4,000 people saved their own lives by fleeing from the valley, many of them suffering respiration problems that lingered for days. Of the 1,800 or so that were unable to escape the toxic cloud, only a few survived. One survivor was Joseph Nkwain from the village of Subum, who described his experience in a 1999 interview:

“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 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 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 p.m. in the afternoon . . . on Friday. (Then) I managed to go over to my neighbors’ houses. They were all dead.”

The exact death toll was uncertain, but nearly every human and animal within a fifteen mile radius had perished. This included between 1,700 and 1,800 people, about 3,500 head of livestock, and massive amounts of wildlife. Investigators observed that Lake Nyos became reddish-brown over the following few days, just as Lake Monoun had done two years earlier. In addition, all vegetation on the lake’s shores had been mysteriously flattened, as if from an explosion.

Before the Lake Nyos eruption, scientists had still been uncertain regarding the cause of the 1984 deaths, but the devastation left by this second event clearly indicated that violent outgassings were to blame. The United States and several Western European countries sent emergency aid money, medical teams, and scientists to Cameroon to assist the victims and assess the cause of the catastrophe. Analysis of the lake confirmed that it was supersaturated with carbon dioxide, even in the aftermath of the eruption. In fact, the scientists estimated that the event had only released about 2% of the total dissolved gas in Lake Nyos. Meanwhile, the vents on the lake bottom are continuing to charge the water with CO2. Without intervention, the lake could erupt again in as little as a few years.

Since then, the much larger Lake Kivu in Rwanda has also identified as a likely site of periodic outgassings. Its waters contain a much higher than normal concentration of dissolved CO2 and methane, and fossil evidence indicates a massive biological die-off around the lake every 1,000 years or so. Due to the geography of Lake Kivu and the dense population along its shores, a limnic eruption there would likely result in the deaths of about two million people.

Today, sets of large polyethylene pipes constantly siphon the CO2-laden water from the bottoms of lakes Monoun and Nyos, producing carbonated geysers on each lake’s surface. However according to a 2005 report by the US Geological Survey, these scant few pipes are insufficient, and the treacherous lakes will not be rendered safe anytime soon. At present levels of gas concentration, a new eruption could occur at either lake, at any time, without warning.

In hopes of slowing down the progression of global warming, some scientists have suggested that the carbon dioxide pollution expelled by powerplants could be captured and stored before it enters the atmosphere; at which point it could be liquefied, and pumped deep in the ocean. There, they hope that the extreme pressure will prevent the CO2 from rising back to the surface. A researcher at the University of Michigan named Youxue Zhang has performed experiments that indicate that the minimum safe depth to prevent CO2 from rising to the surface would be about 800 meters down⁠—about half a mile⁠—but possibly as much as 3,000 meters, which is just short of two miles. According to Zhang, “Droplets injected to a depth of 800 meters will rise, but if they are small enough they should dissolve completely before reaching the liquid-gas transition depth⁠⁠—assuming everything works perfectly.”

Of course all of these data are based on computer models, and cannot possibly account for every possible variable, so one cannot have complete confidence in the results. Not does it account for what will happen if there are problems with the apparatus. Perhaps this does afford a way to dispose of some excess CO2 and slow down the progression of global warming, at least in the short-term. It may serve as a stop-gap while measures are taken to reduce overall carbon dioxide pollution. But trying to bury away our troubles in the oceans is certainly a problematic and irresponsible approach. Considering the deadly examples of lakes Nyos and Monoun, one hopes that humankind will not be foolish enough to adopt this solution in the long term.

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