Kvarken (alternative spelling Quarken, Swedish Kvarken, Finnish Merenkurkku, or North Kvarken as opposed to South Kvarken) is the narrow region in the Gulf of Bothnia separating the Bothnian Bay (the inner part of the gulf) from the Bothnian Sea. The distance from Swedish mainland to Finnish mainland is around 80 km while the distance between the outmost islands is only 25 km. The water depth in the Kvarken region is only around 25 meters. The region also has an unusual rate of land rising at about 1 cm a year. On the Finnish side of Kvarken, there is a large archipelago. Most of the small islands are inhabited. The archipelago is smaller on the Swedish side of the region, and the islands have much steeper shores. The Kvarken region was historically important also, because mail was delivered across Kvarken when the sea was completely frozen from the Swedish to the Finnish coast. This mail route was used frequently during the period of Swedish rule. In 2006, the Kvarken Archipelago was added as an extension to the World Heritage Site of the High Coast (located on the western shore of the Gulf of Bothnia) in Sweden, because it is "continuously rising from the sea in a process of rapid glacio-isostatic uplift, whereby the land, previously weighed down under the weight of a glacier, lifts at rates that are among the highest in the world. As a consequence of the advancing shoreline, islands appear and unite, peninsulas expand, lakes evolve from bays and develop into marshes and peat fens. This property is essentially a "type area" for research on isostasy; the phenomenon having been first recognized and studied here." Most of the World Heritage Site is situated in the Korsholm municipality.