Lake Nyos sits high in the hills of north-western Cameroon, filling the crater of an old volcano. It's deep, quiet and blue.

On the night of 21 August 1986, people in the villages below heard a rumbling. Some saw a fountain of water rise from the lake. Then, silently, an invisible cloud poured over the crater's rim and flowed downhill into the valleys.

By morning, 1,746 people were dead. So were thousands of cattle. Birds and insects had died too. Many victims were found where they had been sitting or sleeping, with no sign of a struggle.

An invisible killer

The cloud was carbon dioxide. CO₂ is heavier than air, so instead of rising and dispersing, it hugged the ground and flowed like a river of invisible water, pooling in low places and displacing the oxygen. Anyone inside it suffocated within minutes.

How does a lake explode?

Lake Nyos sits above volcanic activity. Carbon dioxide seeps up from deep underground and dissolves into the lake's bottom water. Because the lake is deep and very stable, its layers rarely mix, so the gas built up and up under pressure, like a sealed bottle of soda.

Defusing a lake

Gas kept building up again. So scientists came up with a remarkably simple fix: since 2001, pipes have been installed reaching deep into the lake. Once the flow is started, the gas-rich water rises up the pipe on its own and sprays harmlessly into the air like a fountain, slowly releasing the pressure.