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Permian-Triassic Extinction

The Permian-Triassic extinction, which took place 251.4 million years ago, was the Earth's largest, with 96% of all marine species and 70% of land species dying off. It's also the only known mass extinction of insects. Possible causes:

Methane Hydrate Gasification: the ocean sea-floor contains large amounts of methane hydrates, if those were to be released by a temperature change, the global temperature would increase by 6 degree Celsius (10.8F).

Flood Basalt Eruptions: Siberian Traps was one of the largest volcanic eruptions in Earth's history, with over 2 million square kilometers covered in lava.

Impact Event: Major asteroid collision with Earth has been proposed as a possible cause but no large impact crater has been located within the right time-period; however, if the asteroid impacted an ocean, the crater would have been destroyed by "conveyor belt" sea-floor spreading. The oldest sea-floor is only 200 million years old.