The underlying geology, harsh climate, and long history of human occupation have shaped the natural heritage of Caithness. Today a diverse landscape incorporates both common and rare habitats and species, and Caithness provides a stronghold for many once common breeding species that have undergone serious declines elsewhere, such as waders, water voles, and flocks of over-wintering birds. Many rare mammals, birds, and fish have been sighted or caught in and around Caithness waters. Harbour porpoises, dolphins (including Risso's, bottle-nosed, common, Atlantic white-sided, and white-beaked dolphins), and minke and long-finned pilot whales are regularly seen from the shore and boats. Both grey and common seals come close to the shore to feed, rest, and raise their pups; a significant population over-winters on small islands in the Thurso river only a short walk from the town centre. Otters can be seen close to river mouths in some of the quieter locations. Much of the centre of Caithness is known as the Flow Country, a large, rolling expanse of peatland and wetland that is the largest expanse of blanket bog in Europe. Around of the Flow Country is protected as both a Special Protection Area (SPA) and Special Area of Conservation (SAC) under the name Caithness and Sutherland Peatlands, and a portion is further designated as the Forsinard Flows national nature reserve. In 2014 of the eastern coastline of Caithness between Helmsdale and Wick was declared a Nature Conservation Marine Protected Area under the title East Caithness Cliffs. The cliffs are also designated as both a Special Protection Area and a Special Area of Conservation. The Caithness landscape is rich with the remains of pre-historic occupation. These include the Grey Cairns of Camster, the Stone Lud, the Hill O Many Stanes, a complex of sites around Loch Yarrows and over 100 brochs. A prehistoric souterrain structure at Caithness has been likened to discoveries at Midgarth and on Shapinsay. Numerous coastal castles (now mostly ruins) are Norwegian (West Norse) in their foundations. When the Norsemen arrived, probably in the 10th century, the county was inhabited by the Picts, but with its culture subject to some Goidelic influence from the Celtic Church. The name Pentland Firth can be read as meaning Pictland Fjord. Numerous bands of Norse settlers landed in the county, and gradually established themselves around the coast.