You've heard of the Great Wall of China. You might even have heard of Hadrian's Wall in England. We know why they were built. We have no idea why anybody created the Khatt Shebib, which is a wall in Jordan that stretches on for 93 miles.
Somewhere between 1500 and 2000 years old, nobody had noticed the wall until a British diplomat sighted it by plane while flying over Jordan in 1948. It's not tall enough to be a defensive wall, so it could possibly have had an agricultural purpose.