The home of many of Shetland's 'tows' (trolls) and giant 'Saxis' of folklore, Unst is the most northerly inhabited island of the British Isles. Rectangular in shape and with a deeply indented coastline, it has an area of 12,068 ha (29,820 acres). Unst, with its cliff scenery and peat moorland, is ecologically one of the most interesting of the Shetland group in the Northern Isles. Rare species of plant such as the Norwegian Sandwort and the Shetland Mouse-ear Chickweed have been found in the Keen of Hamar National Nature Reserve, the former being discovered by locally-born Thomas Edmondston, author of "A Flora of Shetland" published in 1845. In the far north Hermaness National Nature Reserve protects 183m (600 feet) cliffs which are home to many species of sea bird. Rising to 285m (935 feet), Saxa Vord is the highest point on the island. At the Ministry of Defence installations around Saxa Vord an un-official wind speed record of 177 mph (285 kph) was recorded in 1962. Sites of archaeological interest include the excavated Norse settlement of Sandwick and Bordastubble, the island's largest standing stone. Buildings of note include the ruined Muness Castle, Britain's northernmost castle, built in 1598 by the tyrannical Laurence Bruce of Cultmalindie in Perthshire, and the mediaeval Kirk of Lund which was last used for worship in 1785. The island's chief settlement is Baltasound and to the northeast at Haroldswick is Britain's most northerly post office from where mail carries a special postmark. Britain's northernmost dwelling house, a croft farmhouse, is further north still at Skaw. With the closure of RAF Saxa Vord, most of Unst's population are crofters and fishermen, with tourism making an increasingly important contribution. The loss of military personnel has brought a sharp fall in population from stable figures of 1148 (1961), 1124 (1971) and 1140 (1981) to 1055 (1991), 720 (2001) and only 632 in 2011.
The island can be reached by car ferry to Belmont from Gutcher on Yell, or by air to Baltasound Airport. It is said that Unst was the influence for Robert Louis Stevenson's novel Treasure Island and the author certainly visited in 1869 with his father, the lighthouse engineer Thomas Stevenson (1818-87). Geologically diverse, soapstone was quarried on the island, and a talc quarry still operates.