The only warning was a nocturnal rumble that resembled distant thunder. Then a silent plume of colorless gas shot up from the turbulent depths of Lake Nios, just inside Cameroon's northwest border. Within minutes, the heavy fumes of carbon dioxide burst over the rim and sank into the valley below, enveloping sleepy hamlets in a deadly bubble. Villagers who had already bedded down for the night quietly suffocated in their sleep.

Others tried to outrun the deadly cloud, overturning tables, chairs and cooking pots as they fled their mudbrick huts. Some desperately stripped off their dresses and shirts to escape the...