The over 1,700-foot "fully operational" tunnel runs from Tijuana to an industrial warehouse in San Diego, California. Authorities said it contains a "sophisticated" structural system that was likely used to smuggle heroin, methamphetamine and fentanyl into the U.S.
"It’s estimated to be about 1,744 feet long, 61 feet deep, with a diameter of about 4 feet at its widest point," said Randy Grossman, U.S. Attorney for the Southern District of California. "It has reinforced walls, a rail system, and electricity for ventilation."
U.S. authorities did not say how long the tunnel had been operating but announced $25 million worth of drugs were seized from the tunnel and its operatives over the weekend. The drugs included 1,762 pounds of cocaine, 165 pounds of meth and 3.5 pounds of heroin.
Six Southern California residents were arrested and charged with conspiring to distribute cocaine, officials said.
The industrial warehouse where the tunnel lets out is near the U.S. Customs and Border Protection Otay Mesa Port of Entry, but U.S. authorities have not commented further on the discovery, Fox 5 reported.
The Department of Homeland Security is investigating the tunnel.
Authorities said the tunnel was discovered as officials began sweeping the area for drugs. They conducted drug raids on several homes where drugs were known to be stashed and then checked the warehouse, where they discovered the tunnel.
At least 15 of these sophisticated tunnels across the border have been discovered since 2006, officials said.
Authorities declined to link the latest tunnel to any specific cartel. By federal law, U.S. authorities must fill the U.S. side of tunnels with concrete after they are discovered.
Last week, Border Patrol agents apprehended multiple sex offenders and gang members who crossed the border into the U.S. amid a border crisis that has overwhelmed border security.
The arrests included 15 illegal immigrants convicted or wanted on sex offense charges and three members of the notoriously violent MS-13 gang.
The Associated Press contributed to this report.