The Permian-Triassic extinction, or "Great Dying," was Earth's most severe mass extinction event, occurring about 252 million years ago, wiping out over 90% of marine species and 70% of terrestrial life, triggered primarily by massive volcanic eruptions in the Siberian Traps, leading to runaway climate change, ocean acidification, and widespread anoxia (lack of oxygen). This catastrophic event reshaped life, paving the way for the rise of dinosaurs in the subsequent Triassic period.