The 'US-Mexico border is literally riddled with tunnels'

Donald Trump doubled down on his promises to build a wall along the US-Mexico border, saying, "We will build a great wall along the southern border" in speech on Wednesday evening.

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

But just a few days before Trump's awkward meeting with the Mexican president and his speech, an example of one of the plan's key flaws emerged: a tunnel.

A bi-national inspection of an underground drainage system in Nogales, Arizona, came across a partially constructed tunnel extending from a cemetery in Sonora, Mexico, into US territory.

Inside the tunnel, agents discovered digging tools, power cords, and a gas-powered generator. Hundreds of such passages have been found since the 1990s, many of them around Nogales.

None of this is exceptional.

"Drug traffickers love using tunnels," journalist Ioan Grillo told Business Insider earlier this year. "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, the former head of international operations at the Drug Enforcement Administration and author of "Deal," told Business Insider in April.

"They have to move those drugs across the border and probably the most secure method is through the use of tunnels."

Traffickers have dug tunnels all along the 2,000-mile frontier between the US and Mexico.

The hard-to-detect nature of those passages, and the highly lucrative cargos that pass through them, ensure that there will always be more to find.