The ices of Greenland and Antarctica bear the fingerprints of a monster: a gigantic volcanic eruption in 539 or 540 A.D. that killed tens of thousands and helped trigger one of the worst periods of global cooling in the last 2,000 years. Now, after years of searching, a team of scientists has finally tracked down the source of the eruption.

The team’s work, published in Quaternary Science Reviews, lays out new evidence that ties the natural disaster to Ilopango, a now-dormant volcano in El Salvador. Researchers estimate that in its sixth-century eruption, Ilopango expelled the equivalent of 10.5 cubic miles of dense rock, making it one of the biggest volcanic events on Earth in the last 7,000 years. The blast was more than a hundred times bigger than the 1980 Mount St. Helens eruption and several times larger than the 1991 eruption of Mount Pinatubo. It dealt the local Maya settlements a blow that forever altered their trajectory.

“This is the largest eruption in Central America that human beings have ever witnessed,” says lead study author Robert Dull, a geologist at California Lutheran University. “The importance of the event is even greater, both how the Maya overcame it and how it impacted what happened next.”

The new work helps solve a longtime geologic mystery. Historical accounts that date to 536 describe a dark fog that dimmed the sun and ushered in a wave of crop deaths. Until recently, scholars were open to the idea that these clouds were the remains of an asteroid or comet. But modern data confirms that the event was volcanic—and that it was two volcanoes up to four years apart, not just one.

Ice cores from Greenland and Antarctica show spikes of sulfate, a byproduct of large volcanic eruptions, at 536 and either 539 or 540. The two volcanoes were so large and so violent, they launched sulfur gases and particles miles into the sky. Since this material reflected sunlight away from Earth’s surface, it triggered severe global cooling: One 2016 study found that the volcanoes decreased average global temperatures by as much as 3.6 degrees Fahrenheit. Crops in northern Europe and elsewhere failed, likely triggering starvation and disease. Though its ties to the volcanoes remain tenuous, the infamous Plague of Justinian—which killed tens of millions of people—started in 541, during the worst of the cooling. (Learn more about human-caused climate change, which will cause more than 3.6 degrees Fahrenheit of warming without rapid emission reductions.)

Geologists surmised that the 536 eruption was from a high-latitude volcano—perhaps one in Iceland or Alaska—and that the 539/540 eruption was in the tropics. But the identities of the volcanoes were unknown.

A date with destiny

In the past, some scientists had considered Ilopango a possible suspect for the 536 eruption. It had clearly erupted sometime between the third and sixth centuries A.D., depositing ash and rock across El Salvador to form a rock formation called the Tierra Blanca Joven (Spanish for “young white earth”). But precisely when Ilopango erupted—and how it affected local people—remained uncertain.

In the late 1990s, Dull was a Ph.D student attempting to study El Salvador’s past land use by looking at vegetation in sediment cores. But the more he looked, the more he noticed a whitish layer of ash: the Tierra Blanca Joven. “My advisor said, ‘Eyes on the prize, this volcano thing is a distraction.’ And I just couldn’t avert my eyes,” says Dull. “How many people were killed? How large of an area was affected?”

In 2001, Dull tried to estimate Ilopango’s effects on the local Maya, but he didn’t make further headway until 2008, when two major studies grabbed Dull’s attention. In one, geologists published new evidence that the historical “dust veil” of 536 was caused by a volcano. In the other, researchers announced that the Tierra Blanca Joven extended into marine sediments off the coast of El Salvador. The Tierra Blanca Joven eruption was even bigger than Dull and others thought.

However, pinning down when Ilopango erupted—and whether it could have caused the cooling in the mid-sixth century—became a huge challenge.

To pinpoint the eruption, researchers relied on radiocarbon dating, which takes advantage of the fact that living plants (and whatever eats them) absorb traces of radioactive carbon-14. Dead plants and animals no longer absorb carbon-14, and the trapped carbon starts to decay like clockwork. By counting up the products of this decay, scientists can see when the plant or animal died, a proxy for the age of objects found nearby.

But background levels of carbon-14 naturally vary over time, which can lead to “plateaus”: time periods when organisms that lived decades apart look like they have the same radiocarbon age. Try as they might, earlier researchers couldn’t nail down the Tierra Blanca Joven eruption because it fell within a 120-year plateau spanning the fifth and sixth centuries A.D.

Earth 101

One way to avoid plateaus is to find trees within the sediments of interest. With any luck, the trees lived part of their lives before or after the plateau. If so, they could yield data that would anchor the tree’s age—and the sediment’s age in turn.

This was why Dull’s team was excited to hear about a quarry about 10 miles from Ilopango, where workers digging for road fill had found intact trees within the Tierra Blanca Joven. At the time of the eruption, gently falling ash had entombed the trees where they stood. “One looked like a freshly fallen tree,” says Dull. “The preservation conditions at that site are some of the most remarkable I’ve ever seen.”

Because the trees were so well preserved, Dull and his team could figure out how old they were when the Ilopango eruption killed them. These new data—a hundred new radiocarbon dates—point to the trees dying in the first half of the 6th century, most likely in the 530s to 540s.

Dull’s team also revised their estimate of Ilopango’s size, taking into account the thickness and spread of Tierra Blanca Joven deposits. They say that Ilopango may have even dwarfed the 1815 Tambora eruption, a huge volcanic event that ushered in “a year without a summer” because of the global cooling it caused. Ilopango likely launched up to a million tons of sulfur miles into the sky, high enough for stratospheric winds to spread the aerosols worldwide and trigger global cooling.

Only one eruption in ice-core data matches Ilopango’s timing, magnitude, and likely effect on the global climate: the one that struck in 539 or 540. “In 1980, I declared to my colleagues, I am determined to get a good date on this eruption until I die—and finally, we have it,” says study coauthor Payson Sheets, an archaeologist at the University of Colorado Boulder and the first scientist to try radiocarbon dating on the eruption. “I am finally very pleased.”

Recovering from the apocalypse

Beyond the global impacts, Ilopango was an apocalyptic event for the people living in El Salvador at the time, many of whom lived in or around densely populated Maya centers. As many as 40,000 to 80,000 people died in the eruption itself, Dull’s team estimates, overwhelmed by superheated rock and gas that screamed down Ilopango’s flanks.

For those on the periphery, Ilopango’s nightmare was just beginning. Ash would have blotted out the sun, turning day to night. Thatched roofs heavy with ash and rock could have collapsed. Food and water supplies would have been decimated, as feet of ash buried reservoirs and agricultural fields. An additional 100,000 to 400,000 people would have been affected; those who didn’t die of starvation or disease would have been forced to flee, perhaps to less affected Maya centers toward the north in modern-day Guatemala.

“This is a nightmare-scenario eruption,” says Janine Krippner, a volcanologist with the Smithsonian Institution’s Global Volcanism Program who wasn’t involved with the study. “Even with the science and the understanding we have today, this would be a truly terrifying event. ... I can only imagine what [local people] thought was happening.”

The refugee crisis caused by Ilopango may have altered the trajectory of Maya culture. Dull says that precisely how remains ripe for further research—but already, there is room to speculate. During the sixth century, Maya centers entered what some scholars describe as a “hiatus” of monument construction. Some Mayanists question whether the hiatus is real, but if it is, could it be a sign of a society busy with the aftereffects of Ilopango?

The Ilopango eruption also roughly coincides with a clear shift in the Classic Maya period. Far from crippling the Maya as a whole, centers in what are now Guatemala and Mexico thrived afterward. From the mid-sixth century until 900, a period called the Late Classic, some Maya centers grew in size and influence, and conflict among them increased. It’s possible that incoming refugees helped expand some centers’ populations, labor pools, and armies.

“If indeed from 100,000 to 400,000 were going, and a portion of those, if not the majority—since they’re culturally connected to the Maya—were going up in that area, that might have been one of the major factors of the Late Classic,” says Lisa Lucero, an anthropologist at the University of Illinois Urbana-Champaign who wasn’t involved with the study.