"Limnic eruptions are one of the most bizarre natural disasters known. The criteria required for one to occur make them very rare. Lake Nyos is in a very remote area of the Cameroonian jungle. It is not very large, only 1.2 miles by 0.75 miles, but it is quite deep, 682 feet. Under the bed, a magma chamber is leaking carbon dioxide into the water. This changes the water into carbonic acid. Carbon dioxide is 1.5 times denser than air, which is why it will not rise from the bottom of a lake, unless shoved up by another force. There are only three such lakes known on Earth.
On August 21, 1986, the carbon dioxide at the bottom of the lake suddenly erupted all at once, 1.6 million tons of it, and released a cloud of carbon dioxide from the lake. This cloud, being heavier than air, hugged the ground contours, and blew out of the lake at 60 mph, went downhill throughout the area at up to 30 mph, and displaced all the oxygen in several small villages, suffocating between 1,700 and 1,800 people, not counting all their livestock.
The force of the gas expulsion also blew out the lake water itself, in an 80 ft high tsunami that stripped the trees, shrubs, and soil off one side of the shore."