251 million years ago: the great Permian Extinction.
251 million years ago a disaster occurred which caused the extinction of 9 out of 10 species in the sea and a slightly smaller proportion of species on land. In fact, it consisted of two different events with an interlude of 10 million years between them.
Scientists have confidently dated the second event occurring within an interval of 200 thousand years, between 251.2 and 251.4 million years ago.
This catastrophe marks the boundary between the end of the Permian period and the beginning of the Triassic period.
No one knows with precision what caused this event, but it is possible it was a combination of volcanism, a decrease in the content of oxygen in the sea, an increase in temperature, and/or the impact of a meteorite.