Lake Nyos, Cameroon

On the evening of August 21, 1986, a silent, invisible killer raced down the hills around Lake Nyos, taking the lives of nearly everything in its path. Scientists would discover months later, after much research, that a cloud of carbon dioxide rose out of the lake and rolled into a valley below at more than 100 kilometers (60 miles) per hour. The suffocating gas killed nearly 1,800 people, 3,500 livestock, and countless birds and insects. Located in northwestern Cameroon, Lake Nyos sits in a crater on the edge of an inactive volcano in the Oku Volcanic Field. A pocket of magma lies 80 kilometers (50 miles) below the surface. Carbon dioxide from that magma slowly percolates through Earth’s crust with the groundwater and accumulates in the bottom of the lake. Eventually the gas becomes too concentrated and a bubble of CO2 bursts from the lake.