Lake Nyos, in the Oku Volcanic field in Cameroon, is about 1800 metres across and 200 metres deep.

It fills the throat of an old volcano that has been inactive for centuries, but the whole region sits atop geothermal sources of carbon dioxide.

When the gas seeps from the ground, it's usually dispersed very quickly by the wind, so it never reaches toxic concentrations. When the gas erupts from the bottom of the lake the effect is different: carbon dioxide dissolves easily in water, given the right conditions of temperature and pressure; the resulting carbonated water is heavier than normal water, so it sits at the bottom of the lake, held in place by the pressure of the upper layer.

The system is not stable, though: new carbon dioxide is constantly being vented, and the carbonated layer grows.

Nobody knows for sure what triggered the disaster. On the night of August 21, 1986 something disturbed the boundary between the two layers, and a bubble of water saturated with carbon dioxide rose to the surface.

As the pressure dropped, the carbonated water released bubbles of gas; this created an upward-moving column that brought up more and more water from the bottom of the lake, in a runaway reaction.

A frothy eruption, more than 70 metres tall, shot up from the lake, while all the gas accumulated in decades (an estimated 100 million cubic metres) was released in a matter of minutes.

The waters of the lake turned red from the iron-rich sediments from the bottom, and a deadly cloud of carbon dioxide hugged the land and rolled towards the nearby villages.

Carbon dioxide usually displaces oxygen from the lungs, but in high concentrations it is also poisonous as well. The toxic cloud covered everything in a radius of 15 kilometers from the lake, killing 1746 people. The landscape was littered by the bodies of dead animals - only plant life survived.

After the catastrophe, a team of scientists worked to stop the deadly buildup of carbon dioxide. The technical challenges were rather hard, but the basic concept is that a pipe can bring the carbonated water to the surface in a safe, controlled way. On January 30, 2001 a spectacular soda spray jet soared more than 50 metres over the lake.

The system will be turned up at regular intervals, gradually releasing the gas and preventing future disasters.

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