For decades, scientists have debated what caused the globe’s fifth mass extinction, which marked the end of the Cretaceous period 66 million years ago.

Perhaps it was the massive Chicxulub meteorite that crashed into what is modern-day Mexico’s Yucatan Peninsula—kicking up enough dirt and dust into the atmosphere to block sunlight and choke off much of the life on Earth. Or maybe it was volcanic eruptions in what’s now India’s Deccan Traps region, which would have produced vast plumes of ash.

New research published in the journal Science Advances suggests one might have caused the other, triggering an expansive network of underwater volcanoes to erupt, as well.

By comparing a timeline of events between the Chicxulub impact against topographic maps of the ocean floor, scientists were able to determine that underwater volcanoes along thousands of miles of ridges may have unleashed a massive amount of magma from beneath the Earth’s crust—up to 230,000 cubic miles of it. That’s because the impact of the meteorite would have sent seismic waves that pushed tectonic plates apart and against each other—including those underwater. The release of so much magma, in addition to huge amounts of basalt and volcanic gases, would have smothered any life around those ocean ridges, the study states.

“If eruptions at oceanic ridges were enhanced by the Chicxulub impact, then it is more plausible that the Deccan Traps were enhanced as well,” the study states, connecting the Mexican impact to the Indian eruptions.

All together, the impact on the environment—above and below the ocean surface—would have been immense. Scientists have estimated the event, whatever caused it, wiped out as much as three-quarters of the planet’s animals and plants.