 Permian-Triassic extinction event

As usual, blame the Russians.

The Permian–Triassic extinction event, often referred to as the "Great Dying," is the most significant mass extinction event in Earth's history, occurring approximately 252 million years ago at the boundary between the Permian and Triassic geological periods. This event is estimated to have resulted in the extinction of about 90-96% of all marine species and approximately 70% of terrestrial vertebrate species.