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.

The lighthouse on Market Island. Photo credit

The border violation was resolved only in 1981, when a joint Swedish-Finnish survey decided that the best way to do it is to alter the border itself. The piece of Swedish land where the lighthouse stood was swapped for an equal portion of Finnish land. The ownership of the coastline was unchanged so as not to interfere with each country's fishing rights. This resulted in an inverted S-shaped border. The border is marked by a series of 10 holes drilled into the rock, because any border marking above the ground could be blown away by the ferocious storm and waves. Indeed, the weather and sea are so powerful that the island’s very shape is subject to significant change. So every 25 years, a joint survey of the island is conducted to effect any border changes that might prove necessary.

Since 1979, the lighthouse is automated.

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Border markings on the rock of Market island. Photo credit

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Sources: Wikipedia / Basement Geographer / NY Times