Last Gasp: Toxic gas could explain great extinction

9:53am, May 25, 2005

Poisonous gas bubbling up from the deep ocean could have caused the largest extinction of species in Earth's history. A new model describes how hydrogen sulfide gas produced by marine microbes might suddenly have built up in the atmosphere 250 million years ago, poisoning land animals. The same event would have destroyed the planet's protective ozone shield and thus killed many land and marine plants.

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