St. Kilda (Scottish Gaelic: Hiort) is an isolated archipelago 64 kilometres (40 miles) west-northwest of North Uist in the North Atlantic Ocean. It contains the westernmost islands of the Outer Hebrides of Scotland. The largest island is Hirta (the only inhabited island), whose sea cliffs are the highest in the United Kingdom; three other islands (Dùn, Soay and Boreray) were also used for grazing and seabird hunting. St Kilda may have been permanently inhabited for at least two millennia, the population probably never exceeding 180. The entire remaining population was evacuated from Hirta in 1930. Currently, the only year-round residents are military personnel. A variety of conservation workers, volunteers and scientists spend time there in the summer months. It became one of Scotland's five World Heritage Sites in 1986 and is one of the few in the world to hold joint status for its natural and cultural qualities.