<human>:  When did the Permian-Triassic Mass extinction happen?
<bot>: The Permian-Triassic mass extinction happened 252 million years ago. It was the largest extinction event in Earth’s history, and it killed off 90% of marine species and 70% of terrestrial species. The cause of the extinction was probably a massive release of carbon dioxide into the atmosphere, which caused a rapid warming of the planet.