Border tunnels left unfilled on Mexican side pose security risk, officials say

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.

The security lapse is a boon for traffickers, experts say, saving them time and money and reducing their risk of being caught as they haul away dirt.