Märket is a small island, also known as a skerry, in the Baltic Sea between Sweden and Finland. It is remarkable for being the smallest sea island shared by two countries and for it’s unorthodox border. In 1809, at the end of a Swedish-Russian war the island was divided up in the Treaty of Frederik-shamn. At the time the treaty cleanly heaved this unimportant island in two.
Decades later the Russians established a lighthouse on the island. The sea surrounding Märket is dotted with treacherous shoals and navigation in the area is difficult. This lighthouse greatly helped navigation in the Baltic, but it caused a new problem. The lighthouse was erected on the Swedish side of the island and was a low-level border dispute for many years.
Finally in 1981 both countries worked on a solution. They both wanted to share an equal amount of the land area on the island. They also wanted to keep the status quo of the sea border to preserve fishing rights. To meet both these conditions the land border became the bizarre shape that we see today. The Finnish continue to have sovereignty over the lighthouse and the Swedish continue to have the same size of ownership on the island.
Reddit user: Petrarch1603