Greenland is the world's largest island, located between the Arctic and Atlantic oceans, east of the Canadian Arctic Archipelago. Greenland is an autonomous territory within the Kingdom of Denmark. Though physiographically a part of the continent of North America, Greenland has been politically and culturally associated with Europe (specifically Norway and Denmark, the colonial powers) for more than a millennium, beginning in 986. The majority of its residents are Inuit, whose ancestors migrated from Alaska through Northern Canada, gradually settling across the island by the 13th century. Today, the population is concentrated mainly on the southwest coast, while the rest of the island is sparsely populated. Greenland is divided into five municipalities – Sermersooq, Kujalleq, Qeqertalik, Qeqqata, and Avannaata. It has two unincorporated areas – the Northeast Greenland National Park and the Thule Air Base. The latter, while under Danish control, is administered by the United States Air Force. Three-quarters of Greenland is covered by the only permanent ice sheet outside of Antarctica. With a population of 56,081 (2020), it is the least densely populated region in the world. About a third of the population lives in Nuuk, the capital and largest city; the second-largest city in terms of population is Sisimiut, 320 kilometres (200 mi) north of Nuuk. The Arctic Umiaq Line ferry acts as a lifeline for western Greenland, connecting the various cities and settlements. Greenland has been inhabited at intervals over at least the last 4,500 years by Arctic peoples whose forebears migrated there from what is now Canada. Norsemen settled the uninhabited southern part of Greenland beginning in the 10th century, having previously settled Iceland. These Norsemen later set sail from Greenland and Iceland, with Leif Erikson becoming the first known European to reach North America nearly 500 years before Columbus reached the Caribbean islands. Inuit peoples arrived in the 13th century.