A Straw

In Africa there are 3 very rare lakes that build up CO2 in the deep waters and if triggered could erupt. This has happened twice already, one time killing 1700 people.

Transcript:

It started with a bang on the night of August 21st, 1986. And then over the course of the next twelve hours or so nearly all life within a 16 mile radius of Lake Nyos (Nee-ohs) in Cameroon was dead. In the three villages nearest the lake: Nyos, Cha, and Subum, the bodies of people were found lifeless but otherwise unharmed. Some were outside or even on their own doorsteps, but many were inside, some still in bed. They’d gone to sleep and never woke up. Men, women, and children perished, but so did the animals. Whole herds of livestock lay dead in their fields. Cows, goats, and pigs, all dead, but a few chickens survived for some reason. 

When all was told, over 1700 people lost their lives in the event, and over 3000 livestock animals, not to mention the natural wildlife. A farmer who lived on a hill above the lake described the eerie unnatural silence that morning when he ventured down to the water’s shore. No birds singing or insects buzzing. There were no flies on the dead that day, the farmer said. The flies were dead too. 

 It didn’t make sense. What could have caused such a catastrophe? Who or what was responsible? 

Reports from the few survivors started to fill in some of the gaps. Villagers rushed to the hospital, many with respiratory issues, and told of how they heard a bang and moments later they, or their family members or friends, collapsed. People reported smelling gas or rotten eggs before losing consciousness. Some of the survivors had passed out and remained that way for hours before finally recovering. Some of the lucky few. Those less fortunate had struggled for breath in their final moments; their lungs attacked by some unseen agent. 

In the days and weeks that followed the incident, investigators used these bits of information to try and piece together just what happened at Lake Nyos that night. Somehow a large amount of an unknown gas descended on these villages and suffocated all life on the ground. But where could it all have come from? The area was once alive with volcanic activity but has long been dormant and there was no seismic activity detected in the area that night. 

They then turned their attention to the lake itself. Vegetation around the lake had been heavily disturbed. And the lake, normally a brilliant blue, was now a muddy red color. Something big had obviously happened here. Something to bring all that iron rich sediment from the bottom of the deep lake to the surface. And when they started to study water samples from the lake they got a pretty good idea just what that was. 

Lake Nyos was saturated with carbon dioxide, and the deeper they took samples the more saturated it became. Below 600 feet the water was so saturated with CO2 that it became clear just what the problem was. For decades that gas had lay dormant at the bottom of the lake, the pressure slowly building. Eventually the deep waters were so saturated with the gas that all it took was a little push to disturb that delicate balance of pressures and allow it to be released. And with the pressure from below so high, the result was fast and it was violent. 

It’s like when you shake a bottle of champagne and then open it. All of that CO2 is stirred up and immediately tries to escape. The pressure can build so much that it shoots the cork into the air, releasing the bubbly contents, and the gas, everywhere. 

Now, this is not a common thing to happen with lakes. In fact it is extremely rare and is the result of a combination of very particular circumstances. Lake Nyos is what’s called a meromictic lake. And these types of lakes, due in part to their extreme depth and steep sides, as well as other factors, don’t mix throughout the year like other lakes do. Most lakes turn over at least once a year, allowing deep waters and shallow waters to mix and keep the lake relatively homogeneous. 

Meromictic lakes, because they do not mix, take on very different biological properties. The sediments that collect on the bottom are never disturbed and the temperature difference from top to bottom can be vast. But overall there is nothing dangerous about these types of lakes. It’s when you add back in fact that Lake Nyos sits on an extinct volcanic field that you get trouble. 

Deep in the Earth below the lake is a large magma chamber. This magma releases, along with other gasses, carbon dioxide. This CO2 then works its way up through the rock below the lake and eventually into the water itself. Normally this gas would bubble its way to the surface from time to time and be released. But because of how deep this lake is and the fact that it never mixes, the gas gets held down, trapped. The pressure of the water is actually so great that the gas dissolves into it. And, over time, those deep lake waters become super saturated with the CO2 gas.

Now it’s important to remember, the gas does not want to be there. It does not want to be in solution with the water and it does not want to be trapped under the water. It is a gas and gasses want to expand. 

So, back to Lake Nyos. When something disturbed the deep waters of the lake just enough, some of that gas came out of solution in the water. That in turn distubed the water around the escaping gas and more gas came out of solution. It was a chain reaction and it happened so rapidly that soon nearly all of this gas was racing up to the lake’s surface and when it got there it rocketed into the air. The gas carried water up like a fountain, shooting as high as 300 feet. The reaction also likely caused a tsunami wave 80 feet tall to go rushing towards the lake shore. All this in the matter of about 20 seconds. 

As the waters pummeled the shore, the enormous cloud of carbon dioxide released from the lake began to descend on the surrounding area, and this is where the troubles really started. It’s estimated that between one and three hundred thousand tons of CO2 were released into the air, and being heavier than oxygen, it quickly sank to the surface, displacing all breathable air. And it happened very very fast. Those in low lying areas suffered the worst of it. One woman recalled a great wind rushing through her family’s compound of huts, causing her to immediately pass out. Hours later, she was the only one of her thirty plus family members, including her four children, to wake up. 

It was a terrible tragedy. A freak accident. And while the factors that had to come together to make it even possible are very rare, they are not unique. In fact a similar event occurred just 60 miles away at Lake Monoun, and it actually happened two years before that disastrous day at Lake Nyos. 

It was August of 1984 when Lake Monoun (Ma-Noon) exploded, known as a limnic eruption. The event was much smaller but still 17 people were killed from the gas that erupted from the lake. They didn’t fully understand what happened until it occurred two years later at Lake Nyos. After that a lot of research was done on those lakes and they realized something terrible: it could happen again. 

In the years immediately after the explosions, both lakes were already returning to dangerous levels of CO2 saturation. Something had to be done about it. They had to find a way to release some of the gas from the deep waters to prevent the build up. And through science and engineering they did. It’s a process appropriately called degassing and it works by using essentially a giant straw to siphon the saturated water to the top of the lake. Once it was started and the stream of CO2 and water came shooting out, about 150 feet into the air, the process was self-sustaining. Now each year the amount of gas released is enough to not only balance the amount entering the water but even reduce the level currently there. 

Though Lake Nyos is on the right track they are not out of the woods yet. The chances of another limnic eruption like the one that happened in 1986 has been reduced but a big enough catalyst could still set it off again. And what about the other lakes, others like Lake Nyos and Lake Monoun. Well there is currently only one other known lake in the world with a risk of a similar type of event. That is Lake Kivu (Key-voo) on the border of the Democratic Republic of Congo and Rwanda. It is a lake much deeper and nearly 2000 times larger than Lake Nyos. There is also evidence that eruptions have occurred there periodically throughout its history. But unlike Lake Nyos, which had roughly 4000 inhabitants nearby when it exploded, the area around Lake Kivu is home to about two million people. 

Today’s episode was written by me, Cory Greiner. Keepsake is produced and edited by Alex Hoelscher. If you have an idea you’d like us to explore on the show, send us an email at keepsakepod@gmail.com. 

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