A crystal found in a shipwreck could be similar to a sunstone - a mythical navigational aid said to have been used by Viking mariners, scientists believe.
The team from France say the transparent crystal may have been used to locate the Sun even on cloudy days.
This could help to explain how the Vikings were able to navigate across large tracts of the sea - well before the invention of the magnetic compass.
However, a number of academics treat the sunstone theory with scepticism.
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