On the night of 21st August 1986, people in remote villages in Northwest Cameroon were about to end one of their days. Families tucked their children safely into bed, left the remaining chores and went to sleep.
The next day, more than 1700 of them didn't wake up. They were dead. Their bodies showed no signs of visible injuries and nobody had any explanation as to how they died.
When the news of this unusual event spread, it created mass fear and panic among the locals. Villagers suspected it to be caused by the spirit of a woman that lived around the lake, while others suspected a bioweapon attack. However, the truth was far more complex and strange.
The villages were located near Lake Nyos, a lake which sat inside an old volcanic crater. For centuries underground volcanic activity had been leaking CO2 gas into its waters. The gas kept building inside it without any escape.
Then on that fateful night, maybe an earthquake or a landslide, ended up releasing all that built up gas. The exact cause remains unknown. Villagers however, heard strange noises from the lake that night.
Soon, an estimated 300000 tons of CO2 engulfed the villages like an invisible cloud. Since CO2 is heavier than air, it didn't dissipate and many villagers never got a chance to escape. They suffocated in their sleep.
A mystery that started with local folklore and a suspected bio weapon attack ended up having a scientific explanation to it. In 2001, almost 15 years after the initial disaster scientists installed pipes that now lets the gas escape safely.