Constructing an impassable wall along the U.S.-Mexico border would be a tall order — even for master real estate developer and 2016 GOP presidential candidate Donald Trump.

"From a security standpoint, it really is not an intelligent solution," said Eric Olson, associate director of the Latin American program at the non-partisan Wilson Center.

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.

As of now, fences or other physical obstacles cover about one third of the border, areas that can be accessed easily by foot or by car, Cornelius said. Trump wants to wall in the entire border to keep out undocumented Mexican immigrants, who he has called "rapists" and "criminals."

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A wall is "absolutely buildable and can be built for far less cost than people think," Trump insisted in an interview with the Washington Post earlier this week. "It’s not even a difficult project if you know what you’re doing."

But experts believe otherwise.

Besides being prohibitively expensive, a wall would offer no guarantee of fewer undocumented immigrants entering the United States, they say. It would also be a nightmare to attempt to build.

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.

"It's private land," Cornelius said. "Trump can't just go in and build his wall."

In addition, "there's no evidence that the fencing that we have already built is an effective deterrent," Cornelius said. Case in point: Undocumented immigrants are more likely to sneak through legal ports of entry by hiding in cars or using falsified immigration documents versus attempting to sneak in through breached fences in remote desert areas, according to Cornelius.

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A wall would only encourage more of this "no-physical-risk" mode of entry, but "nobody talks about staffing up the legal ports of entry because it's not as sexy of an alternative," he said.

Olson echoed those concerns: "The more difficult it is to traverse the border, the more people rely on organized crime."

A wall wouldn't stave the flow of undocumented immigrants, Olson said, but it would put a damper on the economy — another platform Trump has vowed to champion if he becomes president.

"[A wall] implies that the relationship between Mexico and the United States is all about criminal interaction, and nothing could be further from the truth," Olson said. "There's a billion dollars a day in trade that's going back and forth between the United States and Mexico... Anything that blocks that hurts the United States."

While walls have been built for hundreds of years to segregate populations — from the Great Wall of China to the Israeli West Bank barrier — studies already conducted by the federal government on building a wall have found it not to be cost-effective, Cornelius said.

"Building a continuous barrier across nearly 2,000 miles of the U.S.-Mexico border is simply not feasible," he said. "Otherwise, it would already have been attempted."