The Permian–Triassic (P–Tr) extinction event, informally known as the Great Dying, was an extinction event that occurre 251.4 million years ago, forming the boundary between the Permian and Triassic geologic periods. It was the Earth's most severe extinction event, with up to 96 percent of all marine species and 70 percent of terrestrial vertebrate species becoming extinct; it is the only known mass extinction of insects. Because so much biodiversity was lost, the recovery of life on earth took significantly longer than after other extinction events. This event has been described as the "mother of all mass extinctions".