How many mass extinctions have occurred during earth's history?

There have been five major mass extinctions in Earth's history, known as the "Big Five." These events significantly altered biodiversity and led to the extinction of a large percentage of species. The most well-known of these is the Permian-Triassic extinction, which occurred around 252 million years ago and is believed to have eliminated about 90% of marine species. The most recent mass extinction, the Cretaceous-Paleogene event, occurred approximately 66 million years ago, leading to the extinction of the dinosaurs.