The Saint Kilda (Scottish Gaelic: Hiort) archipelago, in the North Atlantic is at the outermost limits of the British Isles. Situated 66 km west-north-west of North Uist, only Rockall is further away from the British mainland. The entire archipelago is owned by the National Trust for Scotland and is a World Heritage Site. The name Saint Kilda is derived from the Norse word skildir, meaning "shields." Hirta is the largest island in the group, followed by Soay (2 km northwest of Hirta) and Boreray (6 km northeast of Hirta); there are several smaller islets including Dun, Levinish, Stac Lee and Stac an Armin. Saint Kilda had been continuously inhabited since prehistoric times but the population dwindled with emigration to the United States and Australia. The population declined to such a level that the economy broke down. There are no permanent residents today but the main island of Hirta is occupied all year round by the people who work on the military base (now almost entirely a civilian workforce), and scientists who carry out research on the feral Soay sheep population. The archipelago was inscribed as a Unesco World Heritage Site in 1986 and this status was extended to the surrounding marine environment in 2003. It is a breeding ground for many important seabird species including Gannets (the world's largest colony), Puffins and Leach's Petrels. The small island of Dun is home to the largest colony of Fulmars in Britain. The St Kilda archipelago is also the site of the most spectacular sea cliffs in the British isles. The highest point in the archipelago is Conachair at 430 m. The whole north face of Conachair is a vertical cliff over 300 m high, falling sheer into the sea. Boreray reaches 384 m, and Soay reaches 378 m.