Lake Nyos

Lake Nyos is a natural dam of volcanic rock that contains a yellow-green lake water. The lake is located on the side of an active volcano in the Oku volcanic plain along the Cameroon, West Africa. In 1986 the lake was releasing a giant cloud of carbon dioxide, which resulted in sufokasi (lack of oxygen) up to 1800 people in surrounding villages, killing people and creatures living in the village.

And now believed that the first deadly incident will never happen again, because thanks to the international project carried out on this place, that is some autosiphon, "pipe" sunk vertically into the ocean like a soda straw. This has successfully reduced the CO2 levels of Lake Nyos.
And this place is now one of the few lakes in the world's most extreme.