Which mass extinction event is known as the 'Great Dying' and occurred about 252 million years ago?

The Permian–Triassic extinction was the mass extinction event known as the “Great Dying,” occurring about 252 million years ago.

It happened at the boundary between the Permian and Triassic periods and was the most severe known extinction in Earth’s history. Approximately 81% of marine species and about 70% of terrestrial vertebrate species disappeared, along with many insect groups. Trilobites, which had survived earlier crises, became extinct during this event.

The leading explanation involves enormous volcanic eruptions in the Siberian Traps. They released carbon dioxide and other gases, causing rapid global warming, ocean acidification, oxygen depletion in seawater, and widespread ecological disruption. Scientists continue to investigate whether the extinction occurred in one main pulse or several phases.

A common mix-up is confusing it with the Cretaceous–Paleogene extinction, which occurred about 66 million years ago and ended the non-avian dinosaurs. The Great Dying instead marks the transition from the Paleozoic to the Mesozoic Era, and ecosystems took millions of years to recover.