The federal officers were patrolling near the border fence Tuesday morning when they noticed a sinkhole that led to the discovery, according to Immigration and Customs Enforcement officials.
Investigating the pit in the ground, the personnel realized that it was caused by an illicit excavation between the northern edge of Tijuana and the south end of San Diego, ICE spokeswoman Lauren Mack said.
The agents called in members of the San Diego Tunnel Task Force, who determined that the underground passageway was about 220 yards long, had a single opening in a home in Mexico and extended only a short distance into the United States.
It had been outfitted with reinforced walls, electric lighting and a cart-and-rail cargo-hauling system.
The tunnel, tall enough for a person to walk through, had no drugs or other contraband in it. No arrests had been made in the case as of late afternoon, the spokeswoman said.
Both U.S. and Mexican authorities will further investigate, according to Mack.