I recently learned about Australia's dingo fence, which is an absolutely mind-boggling piece of infrastructure. At 3,488 miles, it's the world's longest fence. Constructed in the late 19th century to keep dingoes out of Southeastern Australia, it is still effective today, as that part of the continent is nearly dingo free, with massive consequences for the local ecology. It is amazing the lengths we will go to accomplish our goals.
I'm reminded of another massive construction project for a single-minded purpose - the Great Hedge of India, which was constructed by the British throughout the 19th century. The hedge, originally a wall, stretched across more than 2,500 miles. It was built to stop smugglers seeking to avoid the Salt Tax, and at its peak employed more than 14,000 people. The hedge, at its zenith, grew up to twelve feet tall and fourteen feet thick. Nothing remains of it today.