'A candy store for smugglers': Step inside the million-dollar drug tunnels that 'riddle' the US-Mexico border

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, told Business Insider. "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, and 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.