Photo: Toxic Deep-Ocean Water Triggered "Great Dying"

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Tube worms cluster around a hydrothermal vent in the Pacific Ocean in an undated photo.

This deep-sea ecosystem harbors bacteria that are anaerobic, or do not depend on oxygen for survival. In warmer conditions, anaerobic bacteria can produce toxic hydrogen sulfide.

A new study suggests that a warming of ocean water 251 million years ago spurred toxic upwellings of hydrogen sulfide, which may have been responsible for Earth's largest mass extinction, known as the "Great Dying."

Photograph courtesy C. Van Dover/OAR/National Undersea Research Program (NURP); College of William & Mary

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