Drug Enforcement Administration officials announced the discovery of what they are calling the “longest cross-border tunnel” under the California-Mexico boundary. The structure stretches more than three-quarters of a mile near Otay Mesa, California.

The discovery followed a multi-year investigation involving numerous law enforcement agencies, technology capabilities, and intelligence gathering, according to information obtained from DEA officials.

“As efforts to strengthen security on our Southern Border increase, Mexican drug cartels are forced underground to smuggle their deadly drugs into the United States,” DEA Special Agent in Charge John W. Callery said in a written statement. “The sophistication of this tunnel demonstrates the determination and monetary resources of the cartels. And although the cartels will continue to use their resources to try and breach our border, the DEA and our partners on the Tunnel Task Force will continue to use our resources to ensure they fail, that our border is secure, and that tunnels like this are shut down to stem the flow of deadly drugs entering the United States.”

Officials say the tunnel begins in Tijuana, Baja California, near an industrial district located west of the Otay Mesa Port of Entry. It then stretches 4,309 feet–making it the longest ever found along the Mexico-U.S. Border. The previous record was discovered near San Diego, officials said. It stretched only 2,966 feet and was found in 2014.

The agency worked with the San Diego Tunnel Task Force to find and shut down the structure. Agencies involved included the DEA, U.S. Border Patrol, Homeland Security Investigations, and the U.S. Attorney’s Office.

During the exploration of the tunnel, officials said it averaged more than five feet tall and about two feet wide. The average depth was reported to be about 70 feet below the surface.

The agents also found an offshoot from the tunnel that surfaced about 3,528 feet into the U.S. The main tunnel continues another city block from there before surfacing in the Otay Mesa warehouse district.

“I am thrilled that this high level narco-tunnel has been discovered and will be rendered unusable for cross-border smuggling. I am proud of the tremendous efforts of the Tunnel Task Force and our agents,” USBP Deputy Chief Patrol Agent Aaron M. Heitke added. “The investigation continues, and I am confident that our hard work and dedication to uphold the law will lead to future arrests and seizures.”

No arrests have been made or drugs seized in connection with the tunnel’s discovery.

“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 HSI Acting Special Agent in Charge Cardell T. Morant. “This discovery underscores the importance of the partnerships HSI has with the U.S. Attorney’s Office, United States Border Patrol, Drug Enforcement Administration, and other regional agencies, as collaborative investigations and community outreach are key to combating this type of threat.”