including Broch of Mousa, Old Scatness and Jarlshof, joined the UKs "Tentative List" of World Heritage Sites. History Scandinavian colonisation The expanding population of Scandinavia led to a shortage of available resources and arable land there and led to a period of Viking expansion so the Norse gradually shifted their attention from plundering to invasion. Shetland was colonised during the late 8th and 9th centuries, the fate of the existing indigenous Pictish population being uncertain. Modern Shetlanders still retain the Norse DNA with many family trees showing the Norse patronymic system(-sson/son, -dottir/daughter). Modern DNA studies such as the Viking Health Study are severely flawed as they only account for a tiny fraction of the population. Vikings then used the islands as a base for pirate expeditions to Norway and the coasts of mainland Scotland. In response, Norwegian king Harald Hårfagre ("Harald Fair Hair") annexed the Northern Isles (comprising Orkney and Shetland) in 875. Rognvald Eysteinsson received Orkney and Shetland as an earldom from Harald as reparation for the death of his son in battle in Scotland, and then passed the earldom on to his brother Sigurd the Mighty. The islands converted to Christianity in the late 10th century. King Olaf I Tryggvasson summoned the jarl Sigurd the Stout during a visit to Orkney and said, "I order you and all your subjects to be baptised. If you refuse, I'll have you killed on the spot and I swear I will ravage every island with fire and steel". Unsurprisingly, Sigurd agreed and the islands became Christian at a stroke. Unusually, from c. 1100 onwards the Norse jarls owed allegiance both to Norway and to the Scottish crown through their holdings as Earls of Caithness. In 1194, when Harald Maddadsson was Earl of Orkney and Shetland, a rebellion broke out against King Sverre Sigurdsson of Norway. The Eyjarskeggjar ("Island Beardies") sailed for Norway but were beaten in the Battle of Florvåg near Bergen. After his victory King Sverre placed Shetland under direct Norwegian rule, a state of affairs that continued for nearly two centuries. Increased Scottish interest From the mid-13th century onwards Scottish monarchs increasingly sought to take control of the islands surrounding their seas. The process was begun in earnest by Alexander II and was continued by his successor Alexander III. This strategy eventually led to an invasion of Scotland by Haakon IV Haakonsson, King of Norway. His fleet assembled in