The ninth plague of Egypt was complete darkness that lasted for three days. But in the year 536, much of the world went dark for a full 18 months, as a mysterious fog rolled over Europe, the Middle East and parts of Asia. The fog blocked the sun during the day, causing temperatures to drop, crops to fail and people to die. It was, you might say, the literal Dark Age.

The main sources of that fog, researchers found, was a volcanic eruption. A 2018 report in the journal Antiquity concluded that a volcanic eruption in Iceland in early 536 helped spread ash across the Northern Hemisphere, creating the fog. Like the 1815 Mount Tambora eruption—the deadliest volcanic eruption on record—this eruption was big enough to alter global climate patterns, causing years of famine.

What exactly did the first 18 months of darkness look like? The Byzantine historian Procopius wrote that “the sun gave forth its light without brightness, like the moon, during this whole year.” He also wrote that it seemed like the sun was constantly in eclipse; and that during this time, “men were free neither from war nor pestilence nor any other thing leading to death.”

Accounts like these weren’t taken very seriously until the 1990s, says Michael McCormick, a history professor at Harvard University and co-author of the Antiquity paper. That decade, researchers examined tree rings in Ireland and found that something weird did happen around 536. Summers in Europe and Asia became 35°F to 37°F colder, with China even reporting summer snow. This Late Antique Little Ice Age, as it’s known, came about when volcanic ash blocked out the sun.

“It was a pretty drastic change; it happened overnight,” McCormick says. “The ancient witnesses really were onto something. They were not being hysterical or imagining the end of the world.”

With this realization, accounts of 536 become newly horrifying. “We marvel to see no shadows of our bodies at noon,” wrote Cassiodorus, a Roman politician. He also wrote that the sun had a “bluish” color, the moon had lost its luster and the “seasons seem to be all jumbled up together.”

The effects of the 536 eruption were compounded by eruptions in 540 and 547, and it took a long time for the Northern Hemisphere to recover. “The Late Antique Little Ice Age that began in the spring of 536 lasted in western Europe until about 660, and it lasted until about 680 in Central Asia,” McCormick says.

"It was the beginning of one of the worst periods to be alive, if not the worst year," McCormick told Science.

This period of cold and starvation caused economic stagnation in Europe that intensified in 541 when the first bubonic plague broke out. The plague killed between one-third and one-half of the population in the Byzantine Empire, or Eastern Roman Empire.

There might still be other, undiscovered volcanic eruptions that contributed to the 536 fog, says Andrei Kurbatov, an Earth and climate sciences professor at the University of Maine and another co-author of the Antiquity paper. However, we now know at least one of the reasons people in 536 couldn’t see their own shadows—even at noon.