Throughout history nations have built walls to protect them from invaders, or to keep their people from escaping. China built the Great Wall to protect itself from invaders, but:
The Manchus were finally able to cross the Great Wall in 1644, after Beijing had already fallen to Li Zicheng‘s rebels. Before this time, the Manchus had crossed the Great Wall multiple times to raid, but this time it was for conquest…The Manchus quickly seized Beijing, and eventually defeated both the rebel-founded Shun dynasty and the remaining Ming resistance, establishing the Qing dynasty rule over all of China.
Then we have the Berlin Wall, and the Inter-German border, effective border controls, with some escapes to a brave and hardy few, but they only lasted 28 years, and were taken down by the very people they were supposed to keep in place.
The United States Southern border is 1,954 miles long. Of that approximately 700 miles is covered with border walls of various shapes and sizes, and the rest of the border the terrain itself is a barrier. Of course neither walls nor terrain will stop someone determined to cross. Smugglers have dug sophisticated tunnels under the border to bring in their goods.
The tunnels were more than 200m (656ft) long and had lighting and ventilation.
A US agent said their design “wasn’t something that an ordinary miner could have put together”. The first tunnel, discovered by the Mexican army on Wednesday, was still unfinished. It started underneath a bathroom sink in a warehouse in Tijuana.
The second tunnel led from an ice plant in the Mexican town of San Luis Rio Colorado to a storage room in what officials described as a “nondescript building” in San Luis, Arizona.
It ran beneath the border at a depth of 16m (55ft) and was lined with plywood.