Mexican drug cartels have burrowed dozens of tunnels in the last decade, outfitted them with rail and cart systems to whisk drugs under the U.S. border and, after being discovered by authorities, abandoned them.
But some of the illicit passageways live on.
At least six previously discovered border tunnels have been reactivated by Mexican trafficking groups in recent years, exposing a recurring large-scale smuggling threat, according to U.S. and Mexican law enforcement officials.
The breaches of border defenses, most recently in December, occur because Mexican authorities, unlike those on the American side, do not fill the tunnels with concrete once they have been discovered. Mexican authorities say they lack the funds.
Instead, only the tunnel openings are sealed. That allows traffickers to simply dig a new entry point to access the largely intact subterranean passageways leading to the U.S. border.
Prompted by Sen. Dianne Feinstein (D-Calif.), who called the tunnels a “national security risk,” the agency has filled every large tunnel up to the border ever since, according to Department of Homeland Security officials.
U.S. authorities at the time anticipated that traffickers would reactivate the tunnels, and some recommended that the U.S. consider paying Mexico’s costs of filling the tunnels on its side. But funding sources were never found.
Since 2007, it has cost Customs and Border Protection $8.7 million to fill drug tunnels, according to a 2016 report by the Department of Homeland Security.
Now an estimated 20 large tunnels, constructed before and after 2007, remain largely intact on the Mexican side, according to U.S. and Mexican officials.
When border fencing went up, traffickers moved underground. Since 2006 there have been 148 tunnels built, according to the DHS, most of them in Arizona and California.
The biggest underground threats now come from what border officials refer to as “super tunnels,” which cost millions of dollars to dig and feature sophisticated touches like lighting and ventilation systems that extend for hundreds of yards down wood-beamed passageways.