Almost two years earlier, on the evening of Aug. 15, 1984, Cameroonians about 62 miles (100 kilometers) southeast of Nyos had heard similar rumblings near a lake. The site of this prior explosion, however, was the smaller Lake Monoun. Around 11:30 p.m., CO2 shot out of the lake and sunk into a valley, near a road. As people from the nearby village of Njindoun walked down the road on their way to work before dawn, they entered the cloud, collapsed and died.
By 10:30 a.m. or so, wind had swept the cloud away. A doctor and police officer arrived on the scene to find most of the 37 dead on a short stretch of road, including a man slumped over his motorcycle [source: Sigurdsson].
The Cameroonian government suspected the explosion was an act of terrorism or the result of someone dumping chemicals into the lake. More traditional villagers in Njindoun believed legends that evil spirits periodically left the lake and killed neighboring people. "Probably these legends came about because of gas bursts in the past," says Evans.
Another lake in Africa may be building toward a burst. Lake Kivu, situated between Rwanda and the Congo in the African Rift Valley, is a legitimate worry. It's more than twice as deep as Nyos and can store more gas. Bacteria in the lake are chugging out methane, and CO2 is leaking in from magma below. Sediment layers suggest the lake may have erupted 7,000 to 8,000 years ago, says Varekamp. Because 2 million people live near Kivu, the lake's gas pressure is being monitored. "If that ever were to go, that would be a natural disaster on a scale we haven't seen, except for the tsunamis in 2004," says Varekamp.
There's also Lake Quilotoa in Ecuador, which is rich in CO2, deep and in a tropical climate. "Some scientists consider it a potential analog of Nyos," says Varekamp.
You may be wondering whether any lake can explode. Could it happen to the pond in your backyard? Let's return to our historic lakes to find out.