The Great Hedge of.... India ?

I assure you, this isn't a gag. The British built it as a Customs barrier across India (then called the British Raj) to help enforce the infamous 'Salt tax' (You might remember Gandhi's famous Salt March ?). Construction was believed to have started in or around 1803 (though as Custom houses, instead of a hedge!)

Surprising that it was widely forgotten!

A colleague of mine online shared this (thanks Imperial !).

The hedge was made primarily of Indian plums , was 12 feet high, 14 feet thick and stretched for a good 2,500 miles! It was primarily used to prevent smugglers from smuggling salt from the coastal areas to interior India and beyond (in order to keep the Salt Tax alive). It actually worked for about a century or so, remaining as a barrier from Punjab to the Bay of Bengal.

Though I don't have enough time to post any more right now, I direct you to the Wikipedia page if you're intrigued.