The definition of an island used in this list is that it is land that is surrounded by seawater on a daily basis, but not necessarily at all stages of the tide, excluding human devices such as bridges and causeways. There are four islands joined to the Shetland Mainland by bridges, East Burra, West Burra, Trondra, and Muckle Roe. There is also a bridge from Housay to Bruray. Nowhere in Shetland is more than three miles (5 km) from the sea. Mavis Grind (Old Norse for "gate of the narrow isthmus") is a narrow neck of land little more than 100 metres (328 feet) wide separating St. Magnus Bay and the Atlantic in the west from Sullom Voe and the North Sea in the east.