One of Donald Trump's signature promises is to build a wall along the border with Mexico. He's said it will be big, powerful, tall, beautiful, impenetrable.
"I would build a great wall, and nobody builds walls better than me, believe me, and I'll build them very inexpensively," Trump said June 16, 2015, when he launched his campaign for the White House. "I will build a great, great wall on our southern border. And I will have Mexico pay for that wall."
"Build the wall!" chants became a staple at his campaign events. But there's a lot at play, from getting a foreign sovereign nation to foot the bill to sorting out the specifics of this monumental endeavor.
WHY HE'S PROMISING IT
Trump says stronger border security, particularly a wall, will end illegal immigration and cut the flow of illicit drugs pouring into the United States.
The U.S.-Mexico border stretches nearly 2,000 miles, more than half of it along the Colorado River and Rio Grande. By May 2015, there were about 650 miles of vehicle and pedestrian fencing, according to a March 2016 report from the U.S. Government Accountability Office.
WHAT NEEDS TO HAPPEN
For Trump to carry out his real estate promise, he'll need funding from Congress and/or Mexico's money. He's outlined potential regulatory changes that he believes will persuade Mexico to comply.
Trump's team has said Congress could appropriate funds for the construction of the wall "to make it more speedy, and then having Mexico pay for it after the fact."
To get Mexico to pay for it, Trump has said he would introduce rules so that no immigrant would be able to send money outside the United States unless they show documentation proving their legal status. He would do that by changing rules that regulate money transfer companies and wire transfers.
Trump says Mexico receives an estimated $24 billion a year in remittances from people in the United States and assumes Mexico will protest U.S. attempts to bar undocumented immigrants from sending money to Mexico.
Trump has said a rule modifying definitions of financial institutions and accounts would not go into effect if Mexico makes a one-time payment of $5 billion to $10 billion, which would go toward border security.
HOW MUCH WILL IT COST
Trump's estimates of the wall's price varied throughout the campaign, at times saying $8 billion or $12 billion. He hasn't been decisive about its physical form, either, suggesting it could rise 35 to 40 feet, or 50 feet, or higher.
But he's said a wall doesn't need to run the nearly 2,000 mile border, but only half of that "because we have natural barriers."
Without precise plans, it's hard to determine how much it would cost to build the wall. By some estimates, completing fencing along the border may cost at least $5.1 billion. Others have estimated that securing the remaining approximate 1,300 miles along the border could cost as much as $25 billion.
WHAT'S STANDING IN HIS WAY
If Trump plans to have Mexico refund him the cost of building the wall, then he faces opposition from Mexican President Enrique Peña Nieto, who has said there's "no way" his country would pay for the wall. (Peña Nieto's 6-year term ends in 2018.)
Engineering and immigration experts have told PolitiFact that the federal government would need to acquire private land along the border and compensate owners for it through eminent domain. That process could take years if landowners put up a fight.
"Every piece of land is different," Paul Barkhurst, an eminent domain litigation lawyer based in San Antonio told PolitiFact Texas back in February 2016. "You're talking about a massive project across many, many states. It just depends on how much resources they want to put on it."
A POSSIBLE TIMELINE
Once the wall's specifics are determined, there's a planning period that could go on for at least one year (including terrain surveying and finalization of design issues), plus project bidding, experts say. Environmental impact studies may also be commissioned.
Engineering experts believe it would take years to build a wall, though it's hard to tell exactly how many without detailed specifics. It's also undetermined when and if Mexico would pay for the wall.