Engineers aboard a floating power station on Lake Kivu could only watch nervously as a volcano in the distance erupted violently, sending tremors rumbling through the water beneath them.
It was not the lava shooting from Mount Nyiragongo last May that spooked them but the enormous concentrations of potentially explosive gases within Kivu, one of Africa's Great Rift lakes lying between Rwanda and the Democratic Republic of Congo.
Flanked by rolling green hills tumbling into glassy waters, Kivu is not quite the picture of tranquility it seems, according to Francois Darchambeau from KivuWatt, a company that extracts gas from the lake's waters for electricity.
Thousands of years of volcanic activity has caused a massive accumulation of methane and carbon dioxide to dissolve in the depths of Kivu - enough to prove monumentally destructive in the rare event they were released.
If triggered, a so-called limnic eruption would cause "a huge explosion of gas from deep waters to the surface" and result in large waves and a poisonous gas cloud that would put the lives of millions at risk.
Only three such lakes exist in the world: Kivu, and Lakes Nyos and Monoun in northwest Cameroon. The latter two experienced limnic eruptions in the 1980s, and the bigger disaster at Nyos suffocated more than 1,700 people in a toxic release of carbon dioxide.
But these catastrophes occurred in a rural area, whereas two million people would be at risk of a similar disaster involving Kivu.
In Rwanda and DR Congo, many live in fear of the lake's harmful potential, and stories abound of swimmers disappearing into its depths after being asphyxiated or pulled under.
The lake, however, poses both peril and promise.
KivuWatt, which says this is the only project of its kind, saw an opportunity to tap these abundant gases for energy generation.
A 20-minute speedboat ride is required to reach KivuWatt's floating platform, a compact tangle of pipes and buoys as high as a multistory building moored in the Rwandan part of Kivu.
With a deafening roar, the facility pumps water saturated with carbon dioxide and methane from around 350 meters to the surface.
As it rises the water and gas separate as the pressure changes.
"It is like opening a bottle of soda," says KivuWatt director Priysham Nundah, who described the project as "halfway between a thermal and a renewable energy plant."
The extracted methane is sent through a pipeline to a second facility onshore in Rwanda, where the gas is transformed into electricity. The carbon dioxide is pumped back into the lake at a precise enough depth to ensure the delicate balance is not upset.
Kivuwatt hopes removing methane could over time reduce pressure, possibly lowering the risk of a limnic eruption.
But fears of such a disaster were reawakened when Nyiragongo, an active volcano north of Kivu in DR Congo, roared to life in early 2021. The lava flow killed 32 people and destroyed hundreds of homes as earthquakes shook the region. A second wave of lava pushed deep into the earth under the lake itself.
From their station, KivuWatt's engineers watched the sky turn red. "It was very frightening," says Nundah. "When the rates of earthquakes and the frequency of earthquakes started to rise no one could really say what would happen."
