<human>:  When did the Permian-Triassic Mass extinction happen?
<bot>: The Permian-Triassic mass extinction happened 252 million years ago. It is the most significant extinction event in Earth’s history. It occurred at the end of the Permian period and the beginning of the Triassic period. It is often called the Great Dying, because it wiped out more than 96% of marine species and 70% of land species. It is thought that this mass extinction was caused by a huge meteorite impact that triggered a series of volcanic eruptions, which released massive amounts of greenhouse gases into the atmosphere.