When most people think of lakes, they also think of summer fun and beautiful views. But three lakes in West Africa paint a disturbingly different picture: they are prone to sudden, deadly explosions that instantly kill people, animals, and plants for kilometers around.
Termed “Limnic eruptions”, these explosions are related to volcanic eruptions, with a few key differences, most noticeably that there’s no lava flow or ash cloud. Instead, a massive but invisible cloud of carbon dioxide is released from the depths of the lake, and being heavier than air, the toxic gas makes its way down the valley to low-lying areas, instantly suffocating all in its path. The only warning signs it gives are the loud boom it makes as the gas escapes from the lake, and a strange smell some survivors have reported, which scientists have attributed to associated volcanic fumes such as sulfur.
The first fatal event on the record happened at Lake Manoun, which erupted in 1984 and killed 37 people and animals.
Then two years later, on August 21, 1986, Lake Nyos, located in a much more populated area, erupted and killed 1,700 people and 3,500 livestock. The white-translucent cloud of carbon dioxide was 160 ft high, and 1.6 million tons of carbon dioxide was released. Descending to the villages below, the toxic levels of carbon dioxide (6-8 percent; the normal amount of CO2 in the air is 0.04 percent) caused immediate loss of consciousness and death. In one moment, people were eating and going about their daily lives; the next moment, they were dead on the floor.
The sole survivors were those at higher altitudes – even being on the second level of a bus was enough to keep one group alive, while those who had been inside the bus got out to examine the engine failure (which was unknowingly due to the limnic eruption) and they immediately perished.
These are the only two events on the record, but there is reason to believe this may have happened many times before. Scientists have found evidence of mass extinction at the bottom of the lakes. Additionally, indigenous people in the area held superstitions of the lake and nearby springs being “haunted”, due to all the dead frogs and birds they would find in the area. But we now have a scientific explanation as to why.
The exact cause of these events, however, is uncertain. Scientists have determined that it does take a specific set of conditions for limnic eruptions to occur, which are unique to these lakes. For one, they are located on the Cameroon Volcanic Line (Mt. Cameroon is one of Africa’s largest volcanoes, and last erupted in September 2000). There is also a large magma chamber under these lakes that generates the volcanic gases, which then emanate into the lakes. Because the lakes are so deep (Lake Nyos is more than 200 meters deep, and surrounded by steep cliffs), there is sufficient water pressure to hold the gases at the bottom. And because the climate is tropical, with warm temperatures year-round, the lake waters don’t mix the way they do in seasonal temperatures, which would allow for a slow release of the gases over time.
Instead, the situation is akin to a soda can that’s been shaken and suddenly opened, only on a much bigger, and deadlier, scale.
Scientists still don’t know exactly what triggered the eruption (or popped the top of the can). There may have been an earthquake or volcanic eruption on the bottom of the lake. There may have been a landslide or two that shifted the water at the top of the lake and allowed the gases at the bottom to come up. Or it may even be that days prior of rainfall cooled the surface of the lake enough to cause an overturning.
Without knowing the exact cause, there might be concern that it could happen again, with special concern for Lake Kivu, where two million people live at risk. But measures have been put in place to prevent limnic eruptions from occurring again, and so far they seem to be working, since it hasn’t happened again in over 30 years. These measures include installing vent pipes in the lakes to slowly de-gas them. There is even the possibility of converting these gases to use as fuel within the city – an excellent example of turning a weakness into a strength.