A rare event in which a large amount of gas (usually carbon dioxide but also possibly methane) erupts from deep below the surface of a lake. Being heavier than air, the CO₂ may then spread at ground level across the land surrounding the lake, causing widespread poisoning. Two limnic eruptions have been recorded, at Lake Monoun, Cameroon, in 1984, killing 37 persons, and at Lake Nyos, Cameroon, in 1986, when the release of more than 80 million m³ of CO₂ killed 1700–1800 people. For a limnic eruption to occur the lake water must be saturated with gas. The deep water is held under greater pressure than the overlying water and is also cooler. Any event (e.g. landslide or earthquake) that displaces some of the deep water, reducing the pressure, or raising its temperature, reducing the solubility of CO₂ or CH₄, may trigger a release of gas.