Federal agents have found the "longest illicit cross-border tunnel ever discovered along the Southwest border," Customs and Border Protection announced Wednesday.

Details: The drug-smuggling tunnel stretches for 4,309ft to connect an industrial site in Tijuana, Mexico, to San Diego, per a CBP statement. "It includes an extensive rail/cart system, forced air ventilation, high voltage electrical cables and panels, an elevator at the tunnel entrance, and a complex drainage system," the statement notes.

The passageway is about 70 feet underground and is about 5 feet, 5 inches tall and two feet wide.

After the tunnel was discovered in late August by CBP and its partners, Mexican law enforcement identified the entrance and members of the San Diego Tunnel Task Force (SDTTF) began mapping it, per the CBP statement.

Why it matters: "While subterranean tunnels are not a new occurrence along the California-Mexico border, the sophistication and length of this particular tunnel demonstrates the time-consuming efforts transnational criminal organizations will undertake to facilitate cross-border smuggling," said Cardell T. Morant, acting special agent in charge of Homeland Security Investigations San Diego in the statement.

The next longest tunnel in the U.S. was discovered in San Diego in 2014. It was 2,966 feet long.

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