The waters of Lake Nyos, Cameroon, turned a murky brown following a deadly release of toxic gas in August 1986.

Photo: Thierry Orban/Corbis Sygma View Slideshow __1986: __A deadly cloud of carbon dioxide sweeps down the slopes of an African volcano, smothering more than 1,700 people.

Volcanoes can kill in many ways, but this one is pretty weird. A volcanic lake in the West African nation of Cameroon degassed violently (you could say it burped, or worse) in the middle of the night. Carbon dioxide is odorless and heavier than air. Most of the victims died in their sleep.

Lake Nyos sits in the crater of a volcano that hadn't erupted in centuries ... and probably didn't actually erupt the night of Aug. 21, 1986.

Magma deep underneath the lake releases carbon dioxide into its depths. Lake Nyos is 690-feet deep, enough for the water pressure to keep the CO2 dissolved in the lake water, rather than letting it bubble up and escape to the surface. And the crater rim towers above the lake, blocking winds which could otherwise stir the surface and create convection currents that would circulate the deep, CO2-saturated water upward to areas of lower pressure. The lack of seasonal variation less than seven degrees north of the equator also contributes to the lake's placidity.

Volcanic rumbling or other seismic activity could have triggered the sudden release of the gas that deadly night, but there's no record of any tremors and no evidence that anything shook off the shelves of homes in nearby villages. It's possible the gas at the lake's bottom just got so concentrated that even under pressure it came out of solution and formed bubbles. Once the bubbles started rising, a "chimney effect" would have rapidly siphoned huge amounts of gas to the surface.

The gas burst through the surface with a rumble, generating a giant wave that scoured vegetation from the shores. The CO2 cloud was at least 300-feet high, because it suffocated cattle on hillsides that far above lake level. Iron from the deep water oxidized and stained the lake waters with rust.

Then the gas crept down the mountain valleys, invading homes. It extinguished oil lamps and suffocated people in their sleep. Some who were awakened by the loud gas bubble stood up and lived, because their heads were above the invisible gas near the ground. But many who went outside paid with their lives.

Few survived. Those from neighboring villages who discovered the devastation recalled with terror the legends about evil demons living in mountain lakes.

Had this happened before? Yes, at least on a smaller scale. A CO2 cloud released by Lake Monoun, about 60 miles south, killed 37 people two years earlier. (The much larger Lake Kivu – on the Congo-Rwanda border – harbors not only carbon dioxide, but methane, in its depths.) And Cameroonians frequently find frogs suffocated by CO2 in low-lying mud puddles.

Engineers hope to prevent a recurrence of the tragedy by continuously degassing Lake Nyos. They've sunk a pipe from a floating platform into the depths of the lake. It shoots a geyser of carbonated water high into the air.

Source: Google Earth; National Geographic, September 1987

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