Vikings conquered Europe thanks to an unexpected technological innovation. They learned how to make tar on an industrial scale and used it to waterproof their longships so that they could undertake large-scale, lengthy pillaging trips around Europe – and across the Atlantic, say archaeologists. Norse raiders were the original Boys from the Blackstuff, it transpires.

The discovery is the work of Andreas Hennius, of Uppsala University. In Antiquity, he reports finding critical evidence that shows output from tar pits in Scandinavia increased dramatically just as Vikings began raiding other parts of Europe. These pits could have made up to 300 litres in a single production cycle, enough to waterproof large numbers of ships. “Tar production … developed from a small-scale activity … into large-scale production that relocated to forested outlands during the Viking period,” says Hennius. “This change … resulted from the increasing demand for tar driven by an evolving maritime culture.”

Vikings were traders and warriors who swept “with threshing oar” from Scandinavia to raid Europe in the 8th century. “Behold the church of St Cuthbert, spattered with the blood of the priests of God … a place more venerable than all in Britain is given as a prey to pagan peoples,” wrote the monk Alcuin of York, after Vikings ransacked Lindisfarne monastery in Northumbria in AD793.

By the 11th century Vikings had become overlords of swaths of Britain and had settled in Iceland, Greenland and America, and raided Spanish ports where they were known as “heathen wizards” by the Moors. Proposed factors for this dramatic increase include changes in climate that boosted agriculture, triggering a sharp rise in population that in turn drove their ships to new lands. Others say local kings were competing for prestige and funded raids to bring back treasures to provide proof of their power.

Now Hennius has pitched in with his theory. Tar drove Vikings to be the hammer of the gods in Europe. He says tar has been used for millennia to waterproof boats. It was made in pits filled with pine wood, covered with turf and set on fire. Small domestic tar kilns were found in Sweden in the early 2000s. These dated to between AD100 and 400. But much larger pits were found during road construction and dated to between 680 and 900, when the rise of the Vikings began. They were originally thought to have been used for making charcoal, but Hennius’s investigation has revealed they had a different purpose: tar manufacture.

These kilns are not associated with any inhabited settlements and were situated closer to forests of pine, which was their key ingredient. These were industrial sites used solely to mass-produce tar, he argues. Vikings were then able to sail their longships on raids. Hennius says: “The size of the late Viking-age fleets suggests an extensive and continuous need for the product.”