Märket ("The Mark") is a small 3.3-hectare (8.2-acre) uninhabited island in the Baltic Sea between Sweden and Finland (in the area of the autonomous Åland Islands), which has been divided between two sovereignties since the Treaty of Fredrikshamn of 1809 defined the border between Sweden and the Russian Empire as going through the middle of the island. The skerry is roughly 350 metres (1,150 ft) long by 150 metres (490 ft) wide, and its area is about 3.3 hectares (8.2 acres). It is the smallest sea island shared by two countries.
The name Märket ('the Mark') probably comes from its usefulness as a navigation mark before there were lighthouses. The route between Sweden and Åland has a passage of about 27 kilometres (15 NM) (17 mi) length over open sea. Before the lighthouse, the island and its shallows were dangerous navigational hazards, which seafarers tried to desperately avoid. In 1873, as many as 23 ships were grounded on the Swedish coast and its archipelago trying to avoid Märket, and eight of them were shipwrecked.
There is a lighthouse on the Finnish side of the current border, which has been unmanned and automated since 1979. When it was built by the Russians and Finns in 1885, the island was considered a no man's land, so it was simply built on the highest point on the island. However, this meant it was built on the Swedish part of the island. As a result, the border was adjusted in 1985 so that the lighthouse is now located on Finnish territory. The adjustment was carried out such that no net transfer of territory occurred, and the ownership of the coastline was unchanged so as not to interfere with each country's fishing rights.
The adjusted border takes the form of an inverted 'S', and the lighthouse is connected to the rest of Finland only by a short stretch of land. The border is regularly resurveyed every 25 years by officials representing both countries. The last such joint inspection took place in August 2006.