The U.S. government has built some form of physical barrier along roughly one-third of the 1,933-mile southern border with Mexico.

The border state with the longest boundary—Texas, at about 1,241 miles—is covered by only 115 miles of barriers.

Data obtained by The Daily Signal shows there is plenty of space for President-elect Donald Trump to make good on his campaign promise to build a wall between the U.S. and Mexico.

But according to experts, physical barriers are only one component of border security, and Trump could encounter similar challenges to his predecessors in trying to construct a wall—or more likely, additional fencing—across complicated, unpredictable terrain.

Trump and congressional Republicans say they could use a 2006 law signed by President George W. Bush called the Secure Fence Act that mandated a minimum of 700 miles of “physical barrier” on the southern border without specifying any particular location where it must be built.

That law called specifically for double-layered fencing to be constructed “along not less than 700 miles of the southwest border where fencing would be most practical and effective.”

But the following year, Congress amended the bill to allow Border Patrol the leeway to decide which type of fencing was appropriate in various regions, meaning that not all of it had to be double-layered.

Sen. Kay Bailey Hutchison, R-Texas, who sponsored the amendment to the law, told the San Antonio Express-News at the time that the Border Patrol should have flexibility to design barriers to best meet the nature of the threat.

“Border patrol agents reported that coyotes and drug-runners were altering their routes as fencing was deployed, so the amendment gives our agents discretion to locate the fence where necessary to achieve operational control of our border,” she said.

But legislators such as Reps. Peter King, R-N.Y., and Duncan Hunter, R-Calif., condemned the amendment, arguing that it dramatically weakened the border fence promised in the 2006 bill by allowing exceptions.

“This is either a blatant oversight or a deliberate attempt to disregard the border security of our country,” King told The Washington Times after Congress amended the bill. “As it’s currently written, the omnibus language guts the Secure Fence Act almost entirely. Quite simply, it is unacceptable.”

The amended law was never fully implemented, and it did not set a deadline for the fencing to be built, meaning Trump could pick up where Bush left off.

The incoming administration just needs money from Congress to do it.

“We’re going to build a wall,” Trump reiterated in his first press conference as president-elect on Wednesday. “I don’t feel like waiting a year or a year and a half. We’re going to start building.”

According to Customs and Border Protection, an agency of the Department of Homeland Security, there is currently about 654 miles of fencing along the border. Christiana Coleman, a spokeswoman for Customs and Border Protection, told The Daily Signal 36 miles of that fencing is double-layered and 14 miles have three layers.

Coleman said the fencing consists of roughly 350 miles of single-layer pedestrian fences, most which stand about 18 feet, and 300 miles of low-level vehicle barriers that can be easily bypassed by pedestrians. The fencing is not continuous. The last segment of the fencing was built in 2014, she said.

The government is obligated by statute to reach the 700-mile floor, meaning it must still build nearly 50 additional miles of fencing at minimum.

A recent report by the Congressional Research Service stated that under the law, the government can build beyond the required 700 miles.

Trump has said that he would not seek to build a wall, or fencing, across the entirety of the nearly 2,000-mile border. He said he envisions the wall covering about half the border because of “natural barriers.”

Trump has estimated the cost of the wall to be from $8 billion to $12 billion. Other estimates have put the cost at $25 billion.

Coleman noted that challenging terrain and other considerations require alternative border security tools, such as “virtual fence” technology using towers, manned and unmanned aircraft, and surveillance sensors.

Building a wall or fence in Texas is especially difficult because U.S. citizens privately own about two-thirds of the border in the state, according to a Government Accountability Office report.

The government would have to purchase land from Texans to build on it.

During his confirmation hearing this week, John F. Kelly, a retired Marine Corps general who is Trump’s nominee to lead the Department of Homeland Security, emphasized these constraints, saying that “a physical barrier in and of itself will not do the job,” and added, “it has to be really a layered defense.”

This article has been modified to clarify what kind of barriers have been constructed on the U.S.-Mexico border.