Sweden-Finland Border Mystery: How a Lighthouse Mistake Reshaped an Island

• A small island, Market Island, between Sweden and Finland, saw its border drastically altered due to a lighthouse built in the wrong territory.
• The 1809 Treaty of Fredrikshamn initially drew a straight border through the island, but a lighthouse built in 1885 by Russian rulers was mistakenly on Swedish land.
• In 1981, Sweden and Finland exchanged land to correct the error, resulting in a unique 'S' shaped border on the island.
• The agreement ensured no change in coastline or fishing rights, with the border marked by 10 deep holes in the rocks due to harsh weather.
• The island's shape constantly changes due to sea waves, prompting joint surveys every 25 years to adjust the border; the lighthouse has been automated since 1979.