Some say it's this Beech hedge in Perthshire, England. That would make sense, right? Jolly old England; tea, lorry, loo... hedge.
Turns out that a far grander hedge existed back in mid-to-late nineteenth century Colonial India: a 12-foot-thick barrier of acacia, prickly pear and bamboo that at it's longest stretched over 2,000 miles from the Himalayas to the sea and was patrolled by 12,000 people in order to extract customs taxes on salt and drugs. And pretty much no one knew this ever happened until this fellow named Rory Moxham found a small reference to it in a forgotten manuscript, went to India to try and find it, and published a book about it in 2002. Now at least a few thousand people have heard of it.
I first read about The Great Hedge, or The Customs Line, in the book Salt, by Mark Kurlansky (the guy who wrote the book Cod). So I went to the internet and there are literally no pictures of it and pretty much no information about it that isn't a review or summary of the book. Wikipedia has the most info you'll find.
So, two opressive regimes (The East India Company, and after the 1857 uprising, the British government) built a 2,000 foot long tree barrier in less than 50 years, and no one knows or cares. Imagine if the U.S. built a cholla, prickly pear, and mesquite fence across our entire Mexican border. Although it's not out of the question for Arizona to do that, I feel like it would get noted for its assholery and weirdness.
It seems like anyone who went to Calcutta during those 50-some-odd years would have at least heard about this monumental wall of shrubbery. I know I haven't read much that would possibly make mention of the Great Hedge (The Moonstone, Lord Jim?), but all signs point to no one writing about it. I guess my biggest question is: did contemporary authors know about this and choose not to use it for the sake of not making the British Empire look bad, or was it just not widely known?