The cartels are always looking for a guaranteed way to cross narcotics into the US and a sophisticated tunnel can allow them to cross large-scale loads. Last month we interdicted a tunnel in San Diego and seized more than 10 tonnes of marijuana.

Compare that to sending four guys across with 30 pounds of marijuana on their backs, who have to navigate the mountains, evade Border Patrols and arrange delivery – you can see the difference in profit.

We’ve seen a variety of clever attempts to disguise entrances, everything from pool tables, electric panels, hydraulic bathtubs, elevators. We saw an exit point in the US where each time they came through the opening, they filled it with concrete, painted it and re-carpeted it. That’s attention to detail.