In fact, “it’s ludicrous,” Olson said of the idea, which has become a hallmark of Trump’s campaign.

Experts estimate that building and maintaining such a wall on the 1,954-mile border, which snakes along four huge states, would cost tens of billions of dollars. And the 21,000 border patrol agents currently on duty would be “nowhere near sufficient” to keep close surveillance on all of it, said Wayne Cornelius, director of University of California San Diego’s Mexican migration field research program.

“Any kind of border barrier can be climbed over, and to prevent that type of activity, we would have to have not just drones in the sky, but a lot of boots on the ground,” he said…

Many parts of the border are dotted with rugged terrains and streams, which would be nearly impossible to build on. In other areas, there are protected wildlife refuges and Indian territory. And elsewhere, ranchers would have to agree to sell their land to the federal government — which is unlikely to happen, even as disgruntled as they may be that migrants are using their property as a border crossing area.