Below the bed, a magma chamber is leaking carbon dioxide into the water. This transforms the water into carbonic acid. Since Carbon dioxide is 1.5 times denser than air, it will not rise from the bottom of a lake, unless pushed up by another force. There are only three such lakes known on whole Earth.
On August 21, 1986, the carbon dioxide underneath the lake suddenly erupted all at once, 1.6 million tons of it, and freed a cloud of carbon dioxide from the lake. This cloud, being heavier than air, clasped 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, excluding their livestock.
The force of the gas discharge 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.