In the eighth century, from the chilly European north emerged a nation of seafarer warriors. These tall, blond people were making both war and trade, but for three centuries they terrorized the coasts of Europe with their pillages.

The most amazing was the Vikings' navigational skills. Using their long, slender boats, called drakkars (below), they were able to navigate through frog and on covered sky, even preferred sometimes these conditions to prepare surprise attacks.

Their navigational skills led them till Iceland, Greenland, and even North America, to Labrador's coast, being the first Europeans to have ever reached America, long before Columbus, in 986.

Now, a new study may have solved the puzzle behind their sailing abilities. Vikings could have used a special crystal called a sunstone (photo above) in order to sail the seas even on gloomy weather or fog conditions.

Testing certain crystals aboard a research vessel in the Arctic Ocean, the team from Eotvos University in Budapest discovered that they can be employed to detect the sun's position even on bad weather, a quality that would have permitted to early sailors to assess their position while navigating under a sky covered by clouds or fog.

This discovery supports the 40 years old hypothesis that the Vikings employed birefringence, an unusual optical property of crystal, during bad weather, a theory regarded with much skepticism by many scientists.

The team spent a month in the Arctic recording polarization (how rays of light display different properties in different directions). Polarisation is not detected by the human eye, but it can be seen using birefringent crystals or sunstones.

The sunstones were discovered to work in all but the worst weather conditions, in certain foggy or cloudy conditions.

Birefringence (double refraction) is the splitting of a light wave into two different component waves, an ordinary and an extraordinary one.

"They've shown that even if the sky is full of clouds and moisture, the polarisation of the sunlight doesn't change very much, and that's a real surprise. If you know the time already, then once you know the position of the sun you know what direction you're sailing in", said Professor Michael Berry, a physicist at Bristol University.