The "Great Dying," also known as the Permian-Triassic extinction event, occurred around 252 million years ago and is considered the most severe mass extinction in Earth's history. It is estimated that about 90% of marine species and 70% of terrestrial vertebrate species went extinct during this period. The causes are believed to include volcanic eruptions, climate change, and changes in ocean chemistry. This event drastically altered the course of evolution, paving the way for the rise of dinosaurs and other new life forms in the following Mesozoic Era. The Great Dying serves as a critical point in understanding biodiversity and the resilience of life on Earth.