Lake Nyos sits in a volcanic crater. Carbon dioxide, the gas you breathe out, seeped into its deep water from below. The water's weight kept it dissolved, like fizz in a sealed soda.
In 1986, something pushed deep water upward. With less weight above, the gas fizzed out. Heavier than air, it flowed downhill into villages, pushing away breathable air. About 1,700 people suffocated.
Today pipes let the gas out slowly.