On August 21, 1986, one of the strangest and deadliest natural disasters in recorded history unfolded in northwestern Cameroon. Lake Nyos, a crater lake situated atop a pocket of magma, had been gradually absorbing massive amounts of carbon dioxide leaking from the Earth beneath it. In typical lakes, deep and shallow waters mix seasonally, allowing dissolved gases to escape harmlessly into the atmosphere.
However, Lake Nyos was thermally stratified, trapping roughly several hundred thousand tons of CO2 under immense water pressure in its cold depths. A sudden trigger—likely a small landslide or rockfall—disrupted the delicate balance, causing a rapid overturn of the lake. In a process akin to uncorking a shaken carbonated drink, an estimated 100,000 to 300,000 tons of carbon dioxide erupted from the lakebed at speeds approaching 100 kilometers per hour.
Being denser than air, the invisible, odorless gas spilled over the northern lip of the crater and rushed down the surrounding valleys. Residents of Lower Cha and nearby villages were asleep when the heavy cloud swept over them. Carbon dioxide displaces oxygen instantly; at concentrations above 15 percent, it causes immediate unconsciousness and rapid respiratory failure.
Within hours, over 1,700 humans and 3,500 head of livestock died without a single structure being damaged. To prevent a recurrence, French engineer Michel Halbwachs led an international effort to install controlled degassing pipes in Lake Nyos starting in 2001, gradually siphon off dissolved gas before dangerous pressures can build again.