The Permian–Triassic extinction event, sometimes known as the Great Dying, was an extinction event that occurred 250 million years ago, forming the boundary between the Permian and Triassic geologic periods, as well as the Paleozoic and Mesozoic eras. It was the Earth’s most severe known extinction event, with up to 96% of all marine species and 70% of terrestrial vertebrate species becoming extinct. It took some 10 million years for Earth to recover from the greatest mass extinction of all time, latest research has revealed. There were apparently two reasons for the delay, the sheer intensity of the crisis, and continuing grim conditions on Earth after the first wave of extinction.