U.S. Border Patrol agents have uncovered a sophisticated underground tunnel stretching more than 1,000 feet into the United States from Mexico as part of a drug-smuggling operation in Southern California.

Federal law enforcement in San Diego, California, found a long, cross-border tunnel that linked Tijuana, Mexico, to the U.S. Though incomplete, the tunnel was intended to be a secret avenue for Mexican drug cartels to move narcotics north, unbeknownst to U.S. authorities, Customs and Border Protection announced late Wednesday.

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U.S. law enforcement worked with Mexico on Monday to find the entrance to the tunnel on the Mexico side of the border, discovered inside a house in the Nueva Tijuana neighborhood of Tijuana. The entrance had recently been covered up with freshly laid tile.

The tunnel was intended to have a final exit point inside a U.S. commercial warehouse near the border.

Members of Border Patrol’s San Diego Sector Tunnel Team found the passage in early April. The tunnel ran beneath the Otay Mesa Port of Entry, which is located about eight miles east of the Pacific Ocean.

The tunnel was nearly 3,000 feet long, roughly two-thirds of a mile long. It measured three and a half feet tall by two and a half feet wide. At its deepest point, it was 50 feet below ground.

The underground passageway was equipped with electrical wiring, lighting, ventilation systems, and a track system for transporting large quantities of goods, such as narcotics.

Agents also found barricades that the builders had installed throughout the tunnel to make it more difficult for U.S. authorities to navigate southbound in the tunnel.

“As we continue to strengthen the nation’s air, and maritime border security, it’s not surprising that foreign terrorist organizations would resort to underground routes,” Jeffrey Stalnaker, acting chief patrol agent of the Border Patrol’s San Diego Sector, said in a statement. “Disruption of narcotics smuggling tunnels is critical to protecting American lives.”

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Border Patrol plans to fill the tunnel with thousands of gallons of concrete to prevent it from being used by Mexican criminals.

The tunnel is one of 95 that federal law enforcement has uncovered in the San Diego area since 1993.