Map of the Strange Border Between Sweden and Finland
"It is possible to walk the length of this short island and cross an international border three times. However, despite the strangeness of the border, Sweden and Finland have always agreed on shared ownership of the skerry (a Norse word for rock in the sea) and the serpentine border results from an effort to correct Finland's accidental siting of the island's lighthouse on the Swedish side of the border. The island is otherwise a textbook example of international cooperation and agreement.
Market Island or Reef (Market Fyr in Swedish) is part of the larger Aland Island chain that lies between Sweden and Finland. According to Wikipedia, ownership of the islands has been shared ever since the Treaty of Fredrikshamn in 1809. Prior to the building of the lighthouse, the border between the two countries passed down the middle of the skerry so that half the island's land mass was in Sweden and half was in Finland. When it was discovered that Finland had accidentally built the lighthouse on Sweden's half of the island, a fix was necessary. The resulting zig-zag border allows the lighthouse to be in Finland while retaining the 50/50 split in land area and avoids any changes to the maritime border. Any alterations of that would have resulted in a change in fishing rights… Perhaps the one thing that might have caused strife between the two herring-loving nations."