Lake Nyos, in the highlands of northwest Cameroon, sits in a volcanic crater, and the ground beneath it leaks carbon dioxide into the deep water. The weight of the water above keeps the gas dissolved, the way a cap keeps it in a bottle, and for a long time nothing showed. On the evening of 21 August 1986 something disturbed it, nobody is sure what, and the lake let go. The gas, heavier than air, ran down the valleys, and about 1,700 people died, some as far as twenty-five kilometres away, many of them asleep, with nothing burned or broken around them. There is a pipe now, from the deep water to the surface. It needed a pump only to get started; once the water rises, the gas comes out of it and lifts the rest, so the lake runs a slow fountain on the same force that did the harm. Nobody can stop the gas coming in. The pipe only sees to it that the lake never again has to hold it all.