lake nyos
© Thierry Orban/Corbis Sygma
How Lake Nyos looked after the methane explosion
"A major theory is that oceans can and have produced methane gas, and was responsible for the Permian Mass Extinction. This explosion killed up to 95% of life on earth during that period. A seafloor basin of methane saturated water may have a breaking point. An earthquake, underwater landslide, or even a meteor impact could have triggered this, and suddenly, like shaking a giant soda bottle, the deepest part of the ocean would have exploded in a deadly storm of methane bubbles."

Something similar happened in 1986 in Lake Nyos in the nation of Cameroon in Africa. This created a ground level carbon dioxide cloud that killed many nearby villagers. This is also mentioned in this video. Watch it below: