US federal prosecutors announced on March 23 that authorities had uncovered a 400-yard tunnel between Mexicali, Mexico, and Calexico, California,and 1,350 pounds of marijuana traveling through it.

Just four days later, US border agents in Arizona discovered another tunnel, an incomplete one stretching only 80 feet, reaching into Nogales.

Finding two tunnels under the US-Mexico border in such a short period of time wasn’t just dumb luck.

“Drug traffickers love using tunnels,” journalist Ioan Grillo told Business Insider. “The Mexico-US border is like a block of cheese with holes in it, with tunnels across it.”

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

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Image courtesy of Reuters