On maps, international borders are two-dimensional: they can be traced out on a flat sheet of paper with latitude and longitude. In real life, terrain is three-dimensional, and borders can be as well, like Friedrichstrasse Station during the Cold War, where one could cross from West to East Berlin just by going up a flight of stairs. However, there's only one four-dimensional border in the world, one that moves back and forth not in space, but in time. Pheasant Island — the world’s oldest condominium — sits in the Bidasoa River between France and Spain. It has been administered jointly by both countries since the 1659 Treaty of the Pyrenees was signed. It’s also the world’s smallest condominium, just 1½ acres. What’s unusual about Pheasant Island is that Spain and France don’t share the territory at the same time. By the terms of the treaty, they alternate sovereignty. Every February, France hands it over to Spain, and every August they get it back. This has happened over 700 times. Think of it as joint custody for countries. “Is this your weekend to be French?” "Spain will pick you up after school."
