People have already begun to resettle the area around Lake Nyos, a deep crater lake next to an inactive volcano in the small country of Cameroon, but they may look to the lake with cautious eyes. Overnight, its waters once turned from a brilliant blue to a bloody rust hue, taking more than 1,700 human lives with it — silently.
Around 9:30 p.m. Aug. 21, 1986, the lake "turned over," sending a deadly cloud of poisonous CO2 gas into the air. The gas, being denser than the surrounding air, slunk along the ground, asphyxiating people, wildlife and domestic animals as it moved soundlessly on its way. Many of the victims of the tragedy never even made it out of bed, and literally did not know what had hit them. Others found they inexplicably couldn’t breathe, and collapsed, waking up with strange wounds, if they woke up at all.
Lake Nyos lies on top of a volcanic source, which releases a great deal of carbon dioxide. The carbon dioxide doesn’t make it to the surface of the lake, but instead is dissolved into the water. When the bottom layer of the lake is disturbed, either by a landslide or by large amounts of rainwater, the carbon dioxide is released. Initially, a volcanic eruption was thought to have been the cause of the disturbance beneath the lake, but later scientists came to believe rapid accumulation of rainwater (denser than the warmer water of the lake) sank to the bottom and caused the overturn.
Either way, it virtually erased all the people in three villages and sent thousands more to the hospital.
Now the lake has been fitted with pipelines meant to release the carbon dioxide in its bottom layers slowly and harmlessly, and people have begun to resettle the areas around the lake.
SDSU has a short article explaining the phenomenon too.