“We will build a great wall along the southern border,” President Donald Trump said this summer, months prior to his election.

Since he took office in January, Trump, along with his Homeland Security Secretary John Kelly, have moved forward with plans to construct that southern barrier.

“The wall will be built where it’s needed first, and then it will be filled in. That’s the way I look at it,” Kelly said last month, adding a few days later that it would be a physical barrier that may have fencing in some places.

Trump has promoted the wall as a part of a solution to cross-border crime and to illegal immigration.

And while an above ground physical barrier may disrupt or delay surface movements, smugglers have long used another transportation method that a wall or fence is unlikely to interrupt.

“Drug traffickers love using tunnels,” journalist Ioan Grillo told Business Insider in early 2016. “The Mexico-US border is like a block of cheese with holes in it, with tunnels across it.”

The “US-Mexico border is literally riddled with tunnels,” Mike Vigil, former head of international operations at the US Drug Enforcement Administration,” told Business Insider last spring.