City 40, or what is today known as Ozersk, is a closed city in Russia that the Soviet Union created during the Cold War. City 40 didn't appear on any maps until 1991, allowed no outsiders to enter, and for a long time, no insiders were allowed out. It wasn't until very recently that the Western world has even become aware of Ozersk, and it took a brave documentary crew sneaking cameras through the heavily guarded gates for any footage of the city to escape.