At first glance, nobody would believe this stunningly beautiful lake could harbor such a sinister secret: What lies beneath?

What is the name of this lake and where is it located in the world?

That's Lake Kivu in Africa.

Lake Kivu, situated between the borders of Rwanda and the Democratic Republic of the Congo (Central Africa), presents an unusually beautiful landscape, surrounded by towering volcanic mountains and verdant hillsides adorned with tea, coffee, and banana plantations.

However, deep below its surface lies a terrifying threat: A deadly cocktail of methane (CH4), carbon dioxide (CO2), and hydrogen sulfide (H2S) lurks. If unleashed, this toxic combination could claim the lives of millions living along its shores, commented the Time magazine.

Currently, the gas-rich water is held down by a layer of heavy salts, preventing it from rising to the surface. But this barrier could easily be breached by an earthquake, a volcanic eruption, or even the increasing pressure from the very gases themselves. Lake Kivu is likened to a ticking time bomb.

Nevertheless, in the eyes of scientists and experts, Lake Kivu also holds the potential to provide valuable energy resources for the people living in the surrounding areas. What have they done to transform this deadly lake into an 'energy mine' for human use?

Let's find out!

THE MOST MYSTERIOUS LAKE ON EARTH

Lake Kivu is one of the most peculiar bodies of water in Africa. A set of unusual characteristics makes it an intriguing subject for scientists, as well as a potentially hazardous location and a source of prosperity for millions living nearby.

Kivu isn't like most deep lakes. Typically, when surface water cools - for example, from winter air temperatures or spring snowmelt - the cold, dense water sinks and warmer, less dense water rises from deeper in the lake. This process, called turnover, often keeps the surface of deep lakes warmer than their depths.

But at Lake Kivu, this turnover doesn't occur, leaving it harboring the most unexpected and astonishing things in the eyes of scientists.

Lake Kivu seen in the Democratic Republic of the Congo, photographed on November 25, 2016. Source: REUTERS / Therese Di Campo / File Photo

Stretching across the border between Rwanda and the Democratic Republic of the Congo, Kivu is one in a chain of lakes running along the East African Rift (EAR), where the African continent is gradually being torn apart by tectonic forces.

As a result, it thins the Earth's crust and activates volcanic activity, creating hot springs beneath Kivu, providing it with hot, carbon dioxide and methane-rich water deep beneath the lake bed.

Microorganisms surviving in Lake Kivu utilize some carbon dioxide, as well as organic matter sinking from above, to generate energy, then produce additional methane as a byproduct. The depth of Kivu - over 457 meters at its deepest point - creates such pressure that these gases remain dissolved.

This mixture of water and dissolved gases is denser than water, preventing the turnover phenomenon mentioned above. The deeper water also becomes saltier due to sedimentation from upper layers of the lake and from minerals in the hot springs, further increasing its density.

As a result, according to limnologist Sergei Katsev of the University of Minnesota Duluth (USA), Kivu is a stratified lake with multiple layers of different densities, with only fragile transitional layers between them.

The unique composition of Lake Kivu in Africa prevents the usual mixing seen in other deep lakes, leading to its unusual water stratification. There's a clear density difference between each layer. The distinct transition between two of these layers is depicted here, with the lower, warmer, saltier water below (red) and the cooler, fresher water above (blue). The boundary between the two layers is only a few centimeters thick. Source: KM

Alfred Wüest, a limnophysicist at the Swiss Federal Institute of Technology in Lausanne, further explains: “The water layers of Lake Kivu can be almost separated into two zones: A less dense surface water zone, about 61 meters deep - And below it is a denser saline water zone that itself is further stratified. However, there's still mixing within each water layer, but they don't interact with each other. Just thinking about the entire body of water in Lake Kivu lying there for thousands of years without doing anything already shows the difference of Kivu.”

Lake Kivu is not only a peculiar attraction for scientists. Its unusual stratification and the trapping of carbon dioxide, methane, and H2S in deeper layers of the lake have researchers concerned that Kivu could be a looming disaster.

'GIANT GAS BOMB'

About 2,253 km northwest of Kivu, there's a volcanic crater lake in Cameroon called Lake Nyos, which also accumulates and retains a large amount of dissolved gas - in this case, carbon dioxide - from a volcano vent at the lake bottom.

On August 21, 1986, the deadly potential of that gas reservoir was horrifically demonstrated. Likely due to a landslide, a large volume of water suddenly shifted, causing the dissolved CO2 gas to rapidly mix with the upper layers of the lake and release the 'gas bomb' into the air. The result, a deadly gas cloud suffocated approximately 1,800 people living in nearby villages.

The volcanic crater lake in Cameroon is known as Lake Nyos. Image: United States Geological Survey

Events like these are called limnic eruptions (or CO2 eruptions), and scientists fear that Lake Kivu may have matured for a similar, even more dangerous event.

Because Lake Nyos is a relatively small lake, only about 1.6 km long, less than 1.6 km wide, and just under 213 meters deep. Meanwhile, Lake Kivu stretches 86 km in length and 48 km in width.

Due to its size, Lake Kivu has the potential for a large, catastrophic limnic eruption, where many cubic kilometers of gas would be released into the atmosphere.

Around 14,000 people lived near Lake Nyos at the time of the CO2 eruption; Over 2 million people live in the vicinity of Lake Kivu today, including approximately 1 million residents of the city of Bukavu, in the Democratic Republic of the Congo. If Kivu were to undergo a CO2 eruption, researcher Sally MacIntyre of the University of California, Santa Barbara (USA) says: “That event would be truly catastrophic!”.

This is not just a theoretical concern. Scientists have found what could be evidence of at least one previous CO2 eruption at Lake Kivu, possibly occurring between 3,500 to 5,000 years ago, and there may be even more recent eruptions. Sediment cores taken from the bottom of Lake Kivu have revealed features known as brown layers distinct from the surrounding sediment layers.

Limnologist Sergei Katsev describes these sediment layers as “highly unusual, rich in organic matter”, possibly the result of previous CO2 eruptions of the lake.

Illustration of Lake Kivu experiencing a CH4 gas explosion. Source: Internet

According to scientists, CO2 eruptions can occur for two reasons. If the water becomes fully saturated with dissolved gases, any additional carbon dioxide or methane pumped into the lake will rise and be released into the air.

Eruptions can also be triggered when something disturbs the deep-water layer along with its dissolved gases mixing with upper layers, reducing the pressure on the gases and allowing them to rapidly escape from solution and out into the open, similar to the effect of shaking a can of soda and then opening it.

Furthermore, Lake Kivu is located in a seismic zone, so an earthquake could generate waves in the lake that would mix the water layers enough to release trapped gases.

Climate is also a potential culprit. At least one past eruption detected in Lake Kivu's sediment records was due to drought causing enough water to evaporate from the lake's surface to lower the pressure in the lower layers and release dissolved gases. Lowering water levels during dry periods could also make Kivu susceptible to unusually heavy rains. They could deliver enough sediment runoff from dozens of rivers into the lake to mix the layers together.

Sally MacIntyre suggests that the likelihood of such events may increase as the planet warms. Climate change will bring more rainfall to East Africa, and “it will come in the form of more extreme rainstorms with longer dry periods”.

Another possible cause could be volcanic activity under Lake Kivu or from the surrounding volcanoes, but scientists believe the risk of that is low. An eruption of nearby Mount Nyiragongo in 2002 did not provide enough material to penetrate the bottom layers of Kivu. And modeling studies have shown that the volcano beneath the lake also wouldn't cause a disruption large enough, Sally MacIntyre says.

Regardless of the culprit, the consequences will be the same: Trapped gases released from their dissolved state, creating dense clouds of carbon dioxide and methane, which can displace oxygen and suffocate both humans and nearby wildlife. And if enough methane is released into the air at Kivu, there's also the added risk it could ignite.

Sergei Katsev says Lake Kivu is regularly monitored for signs of increasing gas concentrations, so a sudden rise in water levels “would not surprise us.” Over a dozen seismic stations also monitor activity near the lake in real-time.

In 2001, an effort began to mitigate the risk of another disaster at Lake Nyos by pumping water from the bottom of the lake through a pipe to the surface, where carbon dioxide is safely released into the air. Similar efforts are also underway at Lake Kivu.

TRANSFORMING A 'DEADLY LAKE' INTO A GIANT ENERGY MINE

With increased gas concentration at Kivu's depths, the risk also rises.

Hydrobiologist Alfred Wüest from the Swiss Federal Institute of Technology and colleagues observed that from 1974 to 2004, carbon dioxide levels increased by 10%, but the greater concern at Lake Kivu was methane, which rose by 15 to 20% over the same period.

However, there might be a way to turn the peril at Lake Kivu into a 'reward.' The same combustible gas - capable of causing a deadly natural disaster - also holds potential as a renewable energy source for the region.

For the Rwandan government, methane-rich waters are not only a threat to be minimized but also an opportunity. Like much of Africa, the country faces electricity shortages to meet the growing demands of its population. Extracting an estimated 60 billion cubic meters of methane gas from the lake's depths could power the country at its current rate for 400 years - a prospect that could alter the landscape in a nation with few alternatives to costly diesel fuel imports.

In 2008, Rwanda launched a pilot program to extract methane gas from Lake Kivu for use as natural gas. And in 2019, Rwanda signed a contract to export bottled methane gas.

Moreover, when the KivuWatt project 'flipped the switch' on December 31, 2015, it exceeded expectations, generating over 25 megawatts of energy as planned from day one.

The new KivuWatt power plant - harnessing trapped methane gas deep within Lake Kivu - was constructed by the U.S. energy company ContourGlobal. Source: Werner Krug / ContourGlobal

Clare Akamanzi, CEO of the Rwanda Development Board, told Reuters in 2019 that bottled methane gas would help reduce the locality's reliance on wood and charcoal, fuels predominantly used by households and tea factories in this East African nation with tens of millions of people.

Methane gas is extracted for use as fuel, and CO2 gas is pumped back down to the lake bottom. Sergei Katsev says: 'They take this gas, transport it through pipelines onshore, and burn it like you burn fossil fuels to generate electricity.'

This harvesting may help mitigate the risk of gas buildup in the lake, although it won't be entirely eradicated. However, for a lake with many lurking dangers below, any effort is helpful and acknowledged.

Once KivuWatt reaches a capacity of 100 megawatts, it will make a significant difference for Rwanda, a developing nation aiming for universal electricity access.

Sources used in the article: Time, Reuters, KM