<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 all marine species. It was caused by a series of volcanic eruptions that released huge amounts of greenhouse gases into the atmosphere, causing the Earth to heat up. The extinction is thought to have been caused by the release of carbon dioxide into the atmosphere.