Some 252 million years ago, life on Earth was rocked by the Permian-Triassic extinction event, the most devastating known loss of biodiversity in our planet’s history. Over a period of just 60,000 years, roughly 90 percent of marine species and 70 percent of terrestrial species perished, earning this event its macabre nickname: The Great Dying. Not even Avengers’ villain Thanos dared dream of such a devastating biological reset button.

Scientists have long puzzled over what made the Great Dying so much harsher than other die-offs. That mystery is explored through a new lens in research published on Monday in Nature Geoscience, led by Michael Broadley, a postdoctoral researcher at the Centre for Petrographic and Geochemical Research in Vandœuvre-lès-Nancy, France.

The relentless volcanic activity that formed the Siberian Traps, a region of basaltic rock that stretches across 7 million square kilometers of Northern Russia, has previously been established as a primary driver of the Great Dying. But Broadley’s team corroborated this link by presenting the first detailed estimates of halogen abundance in the lithosphere, Earth’s outer crust, before and after the mass extinction.

Lithosphere sample. Image: Michael W. Broadley.

Halogens are a family of elements that includes fluorine, chlorine, bromine, iodine, and astatine. Some halogens are naturally toxic, and some become especially deadly when combined with other molecules. Broadley and his colleagues studied two samples extracted from the Siberian Traps, dated about 100 million years before and 100 million years after the Great Dying. The sampling suggested that the Siberian lithosphere had been rich in halogens before the extinction event, and was depleted after it concluded.

In other words, the enormous die-off may have been especially severe because the volcanic plumes released into the atmosphere were heavily dosed with toxic halogens. In addition to driving global climate change, these chemicals could have decimated the ozone layer, exposing life on Earth to dangerous cosmic and solar rays.

Read More: Mathematical Formula Predicts Global Mass Extinction Event in 2100

“The eruption of halogens into the stratosphere catalyses ozone-destroying reactions, raising surface levels of biologically damaging UV radiation,” the authors wrote in the paper. This may have set off a chain reaction that “could potentially have liberated major amounts of halogens and other volatiles to the atmosphere, contributing to species decline and extinction during the end-Permian crisis.”

These halogen-rich plumes were only one of many influential factors that determined the ferocity of the Great Dying, but they are a crucial piece of the puzzle. As human beings try to mitigate the global impacts of our own industrial activity, it’s useful to remember that Earth can be a cruel mistress with no help from us.