Extinction level event

An extinction level event (ELE), also referred to as a mass extinction, or just as an extinction event, is an event where the majority of species (mostly animals) on the planet become extinct. This is opposed to normal extinctions of just single species, which happen all the time in the wild, at a background extinction rate. Things that cause extinction level events include supervolcanoes and mass climate change, which are events suspected to have happened in the past on several occasions. The most spectacular and popular, of course, are asteroid impacts-which were popularized in the late 1990s by the movies Deep Impact and Armageddon.

Previous extinction level events
The most famous; undoubtedly because it is among the most spectacular; historical extinction would be the asteroid impact that killed off the dinosaurs (the K-Pg extinction event) 65 million years ago. 50% of plants and animals died out.

However, this pales in magnitude compared to the event, which ended the Permian period 252 million years ago. Also known as the Permian mass extinction or the "Great Dying", it resulted in the worldwide loss of 57% of all families, 83% of all genera, 96% of all marine species, and an estimated 70% of all land species. Its cause is unclear but may have involved gradual climate change made suddenly much worse by meteor impact or volcanoes.

Similar explanations are suggested for the around 201 million years ago.

The 360-375 million years ago only affected aquatic life; again its causes are unclear but oxygen levels in water seem to have dropped dramatically.

Finally, the extinction which is understood most clearly is happening now, the. The cause: people killing animals and other living things — as well as habitat loss, climate change, and poaching/overhunting.

Plausible causes
It's generally hard to figure out what happened last night, let alone 300 million years ago. Explanations vary but the following mechanisms have been advanced for past extinction events:
 * Impacts from meteorites, comets, asteroids: typically causing tsunamis and blotting out the sun with dust.
 * Massive volcanoes, causing either global warming or global cooling.
 * Gradual climate change, which may have been caused by small continents mushing up into big continents (from Plate tectonics) and changing winds and ocean currents, or more complex mechanisms such as a runaway greenhouse effect, etc.
 * Changes in atmospheric oxygen, most obviously due to the widespread proliferation of : the
 * Astronomical effects: dark matter, supernovae, or gamma-ray bursts, any of which could reduce sunlight, fry the planet with radiation, and provoke mass panic and suicide.

Implausible causes

 * Aliens (aliensdidit)
 * God killing stuff (goddidit)