BBCnews is carrying this summary of new developments in research on the great Permian extinction, which saw the demise of perhaps four-fifths* of the Earth's species. According to the article, the latest candidate is poison gas, released in gargantuan quantities during the largest outpouring of lava ever discovered.

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* The article says "two-thirds of reptile and amphibian families", but I've heard/seen/read much higher estimates elsewhere.