New clues in a mass murder that took place 252 million years ago points to a suspect: Ocean acidification may have driven the largest extinction of animals the world has ever seen.

Carbon dioxide belched out by volcanic eruptions during the Permian period could have caused the oceans’ chemistry to change. That’s worrisome because CO2 levels are rising today — thanks to the burning of fossil fuels — and pushing down seawater pH, researchers report online June 8 in Geology.