Lake Nyos. Today, the lake also poses a threat because its natural wall is weakening. A geological tremor could cause this natural levée to give way, allowing water to rush into downstream villages all the way into Nigeria and allowing much carbon dioxide to escape.
Lake Nyos is a crater lake in Cameroon. Crater lakes commonly have high levels of carbon dioxide but on Aug. 21, 1986, something in the lake went off. It literally exploded in what's known as a limnic eruption, sending a fountain of water over 300 feet into the air and creating a small tsunami. Hundreds of thousands of tons of carbon dioxide burst forth at 60 miles an hour, suffocating people up to 15 miles away. Over 1,746 people and 3,500 livestock perished in minutes.