For an 18-month period beginning in the year 536, a persistent, dusty fog shrouded much of the Northern Hemisphere, dimming the sun to the brightness of the moon. Contemporary chroniclers from Ireland to China documented this strange veil, which triggered a sharp and devastating drop in global temperatures, leading to summer snow, catastrophic crop failures, and widespread famine. While modern analysis of ice cores from Greenland and Antarctica strongly suggests a colossal volcanic eruption was the cause, the specific location of the volcano remains debated. For the people who endured it, the source of the sun-dimming haze was an inexplicable and terrifying event that marked the beginning of the coldest decade in the last 2,300 years.