Scientists and historians have long puzzled over how Vikings navigated the mysteries of the Atlantic Ocean without compasses. The Vikings, according to their sagas, simply used magical stones that guided them in the correct direction. Despite the easy explanation, no one has actually found evidence of one of these sólarsteinn, or sunstones.

So, what is a sunstone? From the Viking sagas, historians knew they were some type of stone made from a crystal-like material that helped navigators find the direction of the sun whenever it was hidden. Normally, Vikings used sundials to direct them, but when the sky was overcast or when dense fog hid the sun, they were directionless. Sunstones supposedly changed that. Simply by lifting the stone to the sky, navigators could determine the location of the sun and find their way thanks to how the light filters through the crystal. According to The Independent:

The principle behind the sunstone relies on its unusual property of creating a double refraction of sunlight, even when it is obscured by cloud or fog of the sort that would have been commonplace for the Vikings. By rotating the crystal against the human eye until the darkness of the two shadows were equal, the sun’s position can pinpointed with remarkable accuracy, according to the researchers.

A breakthrough in the mystery came in the 2000s when scientists excavated wreckage from the Alderney, an English warship that sank at the end of the 16th century. As they rummaged through the wreck, divers discovered a thick crystal stone, the size and shape of a cigarette pack. Lying a mere three feet away from navigation dividers, researchers suspected sailors kept the crystal with other navigational tools.

After conducting various tests, researchers discovered the crystal was Icelandic Spar, the material scientists had speculated the stones had been made out of. As they experimented with the stone to see how well it could actually work, scientists realized the sunstone was so accurate, it could find sun’s location within one degree of accuracy.

Researchers say the reason we’ve never found a sunstone in Viking burial sites may be because the Vikings cremated their dead, something that would cause the crystals explode.

(h/t The Independent)