Some 1,700 people living in the valley below Lake Nyos in northwestern Cameroon mysteriously died on the evening of August 26, 1986. Word of the disaster spread, and scientists arrived from around the world. What they discovered was that the crater lake, perched inside a dormant volcano, had become laden with carbon dioxide gas. This gas had suddenly bubbled out of the lake and asphyxiated nearly every living being in the surrounding valley community.

The disaster, however odd, wasn't unique. Two years earlier, Lake Monoun, 60 miles to the southeast, released a heavy cloud of toxic gas, killing 37 people. A third lake, Lake Kivu, on the Congo-Rwanda border in Central Africa, is also known to act as a reservoir of carbon dioxide and methane, a valuable natural gas (it can be sold for a profit) that is gathered from the lake and used locally.

These three lakes are the only ones in the world known to contain high concentrations of carbon dioxide in their waters. More typically, the gas is released into the atmosphere in harmlessly bubbling soda springs, which can be found around the world.