Kvarken ( (as opposed to South Kvarken); ) is the narrow region of the Gulf of Bothnia separating the Bothnian Bay (the inner part of the gulf) from the Bothnian Sea. The distance from the Swedish mainland to the Finnish mainland is around , while the distance between the outermost islands is only . The water depth in the Kvarken region is only around . The region also has an unusual rate of land rising at almost a year. Several attempts to cross the strait swimming have been made, but cold water and currents have usually been insurmountable obstacles. The first successful crossing was carried out by Lennart Flygare, Pavio Grzelewski and Tore Klingberg, who on 24 July, 2018, swam from Valassaaret (Valsörarna) on the Finnish side to Holmöarna in Sweden. It took them 12 hours 2 minutes to cross the strait. Kvarken Archipelago On the Finnish side of Kvarken, there is a large archipelago, the Kvarken Archipelago, which includes the large islands Replot, Björkö and over 5600 smaller islands. Most of it belongs to the municipality of Korsholm. Most of the small islands are inhabited. The archipelago is smaller on the Swedish side of the region, and the islands, part of Umeå Municipality, 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 Sweden–Finland. During the Ice Ages, the Kvarken region was located underneath the Fennoscandian ice sheet. When the ice sheet retreated from the area around 9600 years ago, the land, which had been compacted under the weight of the ice, rapidly expanded in a process known as isostatic rebound. The Kvarken region is known for having some of the fastest rates of this uplift on Earth, having risen 285 meters since glacial retreat. It continues to rise, demonstrating the effects of deglaciation on flat and shallow archipelagos. The Kvarken Archipelago is also notable for its unique moraine formations (called de Geer moraines), which form regular clusters that are parallel to the ice front. In 2006, parts of the Kvarken Archipelago were 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 of its outstanding demonstration of isostatic uplift, and the influence