According to Viking legend, the seafarers found their way in bad weather using glowing sunstones that revealed the position of the sun even when it was obscured by cloud or had sunk beneath the horizon.
Scientists have long argued over whether such a trick is feasible, but new research on a crystal recovered from a 16th century shipwreck reveals such stones could indeed have existed.
Vikings are known to have sailed vast stretches of open water to reach North America more than a thousand years ago, finding their way from the sun and stars, and the direction of the wind, waves and swell.
It has been speculated that Viking sunstones might have been Icelandic spar, a clear calcite that is common in the region. Calcite splits incoming rays of light in two, known as birefringence.
Vikings might have calibrated calcite crystal sunstones by scanning them across a clear sky and noting the sun's position when the crystal brightened. They could then repeat the trick to locate the sun when it was no longer visible.