On the night of May 5th of the year 1110, the full Moon suddenly and mysteriously disappeared from view. According to a contemporary testimony preserved in Anglo-Saxon chronicles, the Moon “completely vanished, nor light, nor its shape, nor anything was visible, despite the night sky was clear with no clouds.” It wasn’t a simple eclipse, as a research team from the University of Geneva concluded, but likely a cloud of volcanic dust blocking the Moon’s light in the higher layers of Earth’s atmosphere.

The ash from volcanic eruptions can cause various unusual atmospheric phenomena, like a brightly colored sky during sunrise and sunset, a pale sun or glowing clouds, as the fine volcanic ash particles scatter or reflect the sunlight. The research team searched for evidence of a volcanic eruption occurring around the time of the mysterious eclipse and its effects on Earth’s atmosphere.

Ice layers preserve sulfur compounds emitted in high concentrations into Earth’s atmosphere during powerful volcanic eruptions. Tree-ring series document the climate during the growing phase of a tree. Large enough volcanic eruptions can cause a global drop in temperatures, slowing down the growth of plants. Combing both records, the research team discovered that a spike of sulfur in ice layers from Greenland coincides with a significant drop in temperatures, almost 1.5°C, on a global scale beginning with the year 1109.

The research also pinpoints a likely volcanic candidate. Historic documents describe the eruption of Mount Asama in Japan in August and October 1108. The ash emitted by the volcano was enough to “cover the garden of the governor, and working on the buried fields became impossible.”

The eruption was, according to the study published in Scientific Reports, one of the most powerful the last 10,000 years and large enough to send large quantities of volcanic dust into Earth’s atmosphere.