Drone aircraft patrol the U.S.-Mexico border from the skies. Fast boats look out for smugglers at sea. And tens of thousands of Border Patrol agents use trucks, horses, all-terrain vehicles and bicycles to stop unauthorized crossers on land.

But there is another route across the border, one in which smugglers slither north. As enforcement efforts have increased and border barriers have been built, tunneling has gained in popularity, with Nogales

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