One dark winter night in 992 C.E.—as the English King Æthelred the Unready was forced to pay tribute to Viking invaders and Almanzor, Muslim ruler of Andalusia, ravaged his Christian enemies in northern Spain—the northern sky was lit up by an ethereal glow. A chronicler in Saxony (modern-day Germany) wrote that “light like the Sun shone from the North.” The heavens were “blood-red,” said another from Ulster in northern Ireland. What they were seeing, according to a new study, was the aurora borealis (an example pictured), or northern lights, reaching much farther south than normal because of an extremely powerful solar flare (known as a superflare) on the sun bombarding Earth with high-energy particles. Several years ago, researchers had noticed that there were spikes in the levels of the radioactive isotope carbon-14 in tree rings found all around the world at the same time. This can be caused by cosmic rays hitting the upper atmosphere and converting nitrogen-14—a normal constituent of air—into carbon-14. But what sort of cosmic rays and where did they come from? Some studies suggested there were reports of unusual aurorae shortly before the biggest carbon-14 spikes in 774–775 C.E. and 993–994 C.E., suggesting the sun as the source of the rays. To find out, scientists scoured written records around the time of the later spike and found eight aurora sightings recorded in Saxony, northern Ireland, and the Korean peninsula, between October 992 and January 993, they report in a study accepted for publication in Solar Physics. From calculations of how far south the aurorae were visible, the team estimates that the solar storm of 992–993 was stronger than any recorded since detailed monitoring began in 1957, but probably not as strong as the famous Carrington event in 1859, which knocked out telegraph networks worldwide. Whereas in the 990s the storm produced little more than a nice light show, if it struck today it would likely devastate our technologically dependent society.