Märket is a small 3.3-hectare lump of rock located in the passage joining the Gulf of Bothnia to the Baltic Sea, between Sweden and Finland. The island is divided between the two countries since the Treaty of Fredrikshamn of 1809 defined the border between Sweden and the Russian Empire which ruled Finland at the time. When the border was drawn by the treaty’s authors, by sheer coincidence, it ran straight through Market Island.
The island lies in the middle of the 11-km-wide and 27-km-long Understen– Märket passage, and was probably used as a useful navigation mark, which is why its named Märket or ‘the Mark’ in Swedish. In order to make the island more useful as a navigational aid, the Russians built a lighthouse on the island in 1885. Accidentally, the structure was erected on the Swedish side of the island.