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The Permian–Triassic (P–Tr or P–T) extinction event, also known as the Great Dying, the End-Permian Extinction or the Great Permian Extinction, occurred about 252 Ma ago (million years ago), forming the boundary between the Permian and Triassic geologic periods, as well as between the Paleozoic and Mesozoic eras. It is considered to be Earth’s most severe known extinction event, with up to 96% of all marine and 70% of terrestrial species becoming extinct. It was a major global event that had profound effects on ecosystems worldwide for many millions of years afterwards.