6. The United States and Canada

Border: Haskell Free Library and Opera House
Area: 8,891 km* (total border length)
Est. Gross Domestic Product: $27.36 trillion (US) / $1.928 trillion (Canada)*
The Haskell Free Library and Opera House, which spans the border between Quebec, Canada, and the U.S. state of Vermont, represents a unique image of mutual understanding and collaboration between two countries. This architectural wonder, which was purposefully built on the international border in 1904, permits guests to wander freely within a shared area without ever having to exit the structure.

the US and Canada /Flickr/richardmcguire
The opera stage and the reading room of the library are located in the United States, while the majority of the audience seats are in Canada, demonstrating a special cross-cultural exchange. But breaching the boundary inside the structure without following the proper channels may result in legal issues, serving as a reminder of the imperceptible borders that separate countries.