A giant cloud of carbon dioxide bubbled up from Lake Nyos in northwestern Cameroon near the border of Nigeria on August 21, 1986, flowing into the valley below. It stripped the air of oxygen, suffocating 1,746 villagers and 3,000 animals to death.

This may sound like a scary movie plot, but for the thousands affected, it was a very real nightmare. A hydrovolcanic eruption 400 years earlier created a crater in the lake, where massive amounts of carbon dioxide built up over the centuries. Volcanic rumblings continued miles below the surface, with gas seeping into the groundwater to create “CO2-charged soda springs” that bled into the lake, says Dr. George Kling, a biologist at the University of Michigan.

When that cap is removed, it explodes like a warm, shaken Coke.
The problem with CO2 is that it builds up, “like in a soda bottle,” Kling says. No one can see the pressure building as long as the cap — in this case, the weight of the water — stays put and forces the gas to dissolve or be submerged. But when that cap is removed, it explodes like a warm, shaken Coke, with bubbles rising to the surface and a giant burst of CO2 escaping into the atmosphere.

Triggering the pressure’s release can be as simple as heavy rain or an earthquake. In Nyos, some boulders and dirt were the likely culprits, and Kling says he found evidence of a “very large and recent landslide” when he visited after the disaster. Dr. Njilah Isaac Konfor, a disaster-management and groundwater expert at the University of Yaounde in Cameroon, says there were reports of a week of rain before the event.

Luckily, there are just three known lakes worldwide with these properties, and only two of these limnic eruptions have occurred in recorded history. The first was a much smaller explosion at Lake Monoun, also in Cameroon, that killed 37. It took place two years earlier, almost to the day, just 19 miles away from Lake Nyos. Volcanologists and engineers, Kling says, have been working to eliminate the CO2 risk there ever since via six-inch pipes that pump out the gas.

While “unlikely,” according to Dr. Greg Tanyileke of the Cameroonian Institute for Geological and Mining Research and the chief government scientist working on the lake, Lake Nyos could suffer another deadly eruption. Warding off such a calamity is an ongoing process, he says. Kling adds that, despite efforts to warn locals and help them make “informed choices,” many have returned to their homes and farmland because the soil there is so fertile, and farming is critical to their rural existence.

But CO2 isn’t the only problem. There’s also a rickety natural dam made of volcanic ash and rock at one part of the crater. If it collapses, the top 130 feet of the lake could come crashing down, causing a flood that “would reach Nigeria” and endanger 5,000 people, Kling says. Even worse, this could cause another imbalance in the lake and possibly unleash any remaining carbon dioxide still suppressed at the bottom.

This “double-whammy threat,” as Kling calls it, is being addressed by an expensive but essential dam-strengthening project that is nearing completion.
Even if Cameroon’s lakes are no longer a threat, scientists have their eyes on a far bigger lake in Central Africa with similar attributes. Lake Kivu, which lies on the border between the Democratic Republic of the Congo and Rwanda, is 1,600 times larger than Nyos and contains 1,000 times more gas. It also sits on the Albertine Rift, where it is enduring increasing volcanic activity.

This, combined with the fact that it has gas levels high enough to kill two million people along its shores if there’s a limnic eruption, makes it what Kling calls the “largest ticking time bomb in the world.”
And scientists haven’t settled on what can be done to avert the danger. The lake is so big that the methods used to degas Cameroon’s bodies of water simply won’t work.

Instead, the Rwandan government is trying to prevent a disaster and capitalize on the natural gas — the fifth largest methane deposit in the world — by carefully harvesting the deposits for electricity. But until this project is done on a larger scale, another disaster like the one at Nyos remains possible.