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.