SAN DIEGO (AP) — U.S. authorities on Monday announced the discovery of a major drug smuggling tunnel — running about the length of a six football fields — from Mexico to a warehouse in an industrial area in the U.S.

The secret passage from Tijuana to San Diego featured rail and ventilation systems, electricity and reinforced walls, authorities said. It was discovered near San Diego's Otay Mesa border crossing in an area where more than a dozen other sophisticated tunnels have been found in the last two decades.

U.S. authorities said it was unknown how long the tunnel had been operating and what amount of drugs, if any, got through undetected. They seized 1,762 pounds (799 kg) of cocaine, 165 pounds (75 kg) of meth and 3.5 pounds (1.6 kg) of heroin in connection with the investigation.

Six people, ages 31 to 55, were charged with conspiring to distribute cocaine. All are Southern California residents.

The tunnel is in one of the most fortified stretches of the border, illustrating the limitations of border walls. While considered effective against small, crudely built tunnels called “gopher holes,” walls are no match for more sophisticated passages that run deeper underground.

The latest passage, discovered Friday, ran one-third of a mile (532 m) to Tijuana. It was 4 feet (1.2 m) in diameter and about six stories deep.

The type of drugs seized may signal a shift from the multi-ton loads of marijuana that were often found in discoveries before California legalized pot for recreational use in 2019.Hard drugs, such as heroin, methamphetamine and fentanyl, are typically smuggled through official border crossings from Mexico because their small size and lack of odor make them difficult to detect. But tunnels give smugglers an advantage of being able to carry huge loads at lightning speed.

Source: Big cross-border tunnel found linking Tijuana, San Diego