Photo: World's Greatest Extinction Not Caused By Toxic Gases

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A map shows Earth approximately 250 million years ago, at the time of the Great Dying extinction that wiped out most existing life. Scientists had blamed the deaths on ozone-destroying hydrogen sulfide released from the sea because of low oxygen levels.

But a new study finds that hydrogen sulfide probably couldn't reach high enough levels in the atmosphere to destroy the ozone layer, prompting scientists to come up with a new explanation for the biological cataclysm.

Image by Nicolle Rager, National Science Foundation, based on Pangaea map data, Paleogeographic Atlas Project, University of Chicago

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