Donald Trump talked a lot about building a wall between the U.S. and Mexico when he was a candidate. Now that he is president, it's unclear just what that wall is going to look like and when it might get built.
What we know so far is that it's likely to be a fence, not a wall, it won't extend the length of the border and it will be very unpopular with its southern neighbor.
At the moment, about a third of the 2,000-mile border with Mexico is fenced. Some parts are 10 feet tall, some are 20 feet, some are wire mesh, some are steel beams.
The oldest kind of fencing is made of landing mat material from the Vietnam War — it's the "primary fence, which is a vehicle barrier" so vehicles won't easily be able to drive from Mexico into the U.S., says James Nielsen, a Border Patrol agent in the San Diego sector.
Border enforcement officials have been telling Trump's transition team that a fence alone is not enough. You also need agents, camera towers, stadium lights and sensors. If the president insists on a "great wall" to deliver on his campaign promise, Customs and Border Protection has identified 400 miles where the fence can be extended or reinforced.
But there are clearly sections where it remains impractical. For instance, the border fence stops at a gnarly mountain range at Otay Mesa about 15 miles east of the Pacific Ocean. The view from here is open country — no dividing line, no nothing.
"At the time that this fence was built, it was too expensive to continue," says Border Patrol spokesman Joshua Devack. The mountains are a natural barrier that, he says, "a lot of crossers choose not to climb. ... They prefer to have access to roads and civilization."
According to an eight-year-old estimate by the Government Accountability Office, the border fence cost the government $3 million to $4 million a mile to build. Estimates for additional fencing — in harsher terrain — could surpass $10 million a mile.
"This is going to divide us more"
Trump still says he will give the bill to Mexico. Earlier this month, Mexican President Enrique Peña Nieto restated that his country "of course" will not pay.
The existing wall and talk of more wall remains unpopular with Mexicans.
As Border Patrol agents are being interviewed, a 15-year-old Mexican girl named Carla Martinez watches from a park on the Tijuana side.
"We are against it," she says into a microphone thrust through a narrow space between the thick iron posts. "We won't pay for it. It's bad because it's racist. There has to be a border between different countries, but this is just going to divide us more."
Mexico has not made the fence-building easy.
In the mid-2000s, Sundt Construction erected about 100 miles of fence in the remote west desert. Company President Mike Hoover says the U.S. government told his construction crews not to set foot across the international divide.
"So we had a tight working area. That was a challenge. It was unusual but with proper planning we could work our way through that," Hoover says.
There were other challenges:
And because some Mexicans were hostile to the fence, Hoover says federal border agents and private security guards had to protect the construction crews around the clock.
"We did have some instances of rock-throwing from the Mexican side. I know of at least one instance where we had some shots fired in our direction, close enough that we were plenty worried," Hoover says.
Despite the challenges, if Trump gets his wall, Sundt Construction hopes to win a new contract and return to the border to continue the international fence.
Correction: January 24, 2017 12:00 am — In the audio, the following quote should have been attributed to Border Patrol spokesman Joshua Devack, not agent James Nielsen: "At the time that this fence was built, it was too expensive to continue. And there's also this natural barrier here that a lot of crossers choose not to climb this mountain. They prefer to have easier access to roads and civilization."