The US-Canadian border along the 49th parallel is the longest national border in the world at 5,525 miles. You'd think such a long border would be fairly invisible, especially since it runs through forests for much of its length. But there's a straight line running through those woods in which a 20-foot-wide swath of trees have been cut down. In fact, they call it the Slash. It's visible in satellite photos.
Stripped of trees, this slice runs through national forests and over mountains. It is too long and remote to be continuously cut down, but every few years (longer on the Western sections, where growth is slower) workers freshly deforest the greenery that grows back.
It might seem unnecessary, but there is a reason for this intervention: a person on either side wandering close to the border can see it and recognize they’re approaching the line. So each year, American taxpayers pay around half a cent each to the International Boundary Commission (IBC) to help periodically maintain this dividing void.