The Haskell Free Library and Opera House was built across the border between the United States and Canada. Yes, there is a thick black line on the floor between the two countries.
Construction began on this building in 1901 and by 1904 it was in use. The intention was to create a shared public building that could be used by both the American residents of Derby Line, Vermont, and the Canadian residents of Stanstead, Quebec. You couldn’t do something like this today – by 1925 treaty you cannot build within 3 metres of the American-Canadian border without special permission from the International Boundary Commission – but at the time it wasn’t a problem.
The building contains (surprise surprise!) a library and an opera house. The opera stage is on the Canadian side, as are all of the books in the library, but the building’s entrance is on the American side. Canadian visitors are allowed to enter via that entrance unencumbered by customs agents, but only if they promise to go back to Canada immediately afterwards. And Canadians never break their promises, so I’m sure the States is safe from invasion. But is Canada safe from the States?
Apparently buildings that are officially in two countries at once are a thing: they’re called “line houses” and there are several of them in Derby Line / Stanstead alone. The Haskell Free Library and Opera House has a big black line running along the border, but no-one cares if you jump back and forth from one side to the other. Well, you might get shushed.
[Thanks to an anonymous reader for suggesting this topic.]