'Like a block of cheese with holes in it' - How Mexican cartels will subvert and avoid Trump's border wall

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.

"They have to move those drugs across the border and probably the most secure method is through the use of tunnels," said Vigil, author of "Metal Coffins: The Blood Alliance Cartel."

US authorities have long relied on old-fashioned detective work to detect tunnels and tunnel construction, supplementing their surveillance with seismic devices and ground-penetrating radar.

But traffickers have gravitated to areas where geographic features, like soil composition, and surface features - such as drainage networks and legitimate construction work - obscure their activities.

The hard-to-detect nature of these passages, and the highly-lucrative cargo that pass through them, ensure that there will always be more to find - something Kelly himself has admitted.