A pale yellow gate stands 4 feet high on a bridge over a rural Arizona irrigation ditch. The nondescript structure epitomizes the challenges of securing the southern border.

Border agents discovered that smugglers were transporting drugs, and perhaps people, across the tiny bridge. After the gate went up, traffic paused until the smugglers figured out they could slip under the poles in a go-kart. Border agents added a lower tier to the gate. Smugglers started using small motorcycles, which they flipped sideways to scoot under the barrier. Border agents welded another section onto the gate, dusting the ground.

That finally stopped motorized traffic here, but it’s likely moved elsewhere. “If there’s a weakness in our infrastructure, they will exploit it,” says Anthony Porvaznik, chief patrol agent for Customs and Border Protection’s Yuma Sector.

Crossing the border illegally is a lucrative business. In 2017 smugglers earned as much as $2.3 billion trafficking migrants from Central America to the U.S., according to a report last month by the RAND Corp. This fiscal year, CBP has $14.69 billion in discretionary funding to cover construction, upgrades and other operational expenses. To secure the border, the agency relies on a combination of manpower, technology and infrastructure. That last category has become controversial, thanks to President Trump’s fixation on erecting a wall.

Even the best of walls don’t keep everyone out, border agents say. The landscape poses a special challenge in the Imperial Sand Dunes, west of Yuma. Sand would bury a typical wall, so the U.S. installed a 16-foot floating wall that can adjust to the shifting landscape. The week I visited, a Honduran migrant had tried to dig under it, but sand seeped in as he shimmied under. Border agents found him flat on his back, trapped with his head and chest in the U.S. and feet in Mexico. They rescued him, and he’ll soon be deported.