U.S. authorities on Wednesday announced the discovery of the longest smuggling tunnel ever found on the Southwest border, stretching more than three-quarters of a mile from an industrial site in Tijuana, Mexico, to the San Diego area.

U.S. Customs and Border Protection said it featured 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.

“This one blows past (the second-longest),” said Lance LeNoir, a Border Patrol operations supervisor. “We never really thought they had the moxie to go that far. They continue to surprise me.”

The tunnel raised questions about the effectiveness of President Donald Trump’s border wall, which stretches several feet underground in the area. The tunnel was found about 70 feet underground, well below the wall.

Following the discovery in August, Mexican law enforcement identified the entrance and members of the San Diego Tunnel Task Force began mapping the tunnel that extends a total of 4,309 feet. (1,313 meters). The next longest tunnel in the U.S. was discovered in San Diego in 2014. It was 2,966 feet (904 meters) long.

The newly discovered tunnel is about 5.5 feet (1.68 meters) tall and 2 feet (0.61 meters) wide and runs at an average depth of 70 feet (21.3 meters) below the surface, officials said.

Agents discovered several hundred sandbags blocking a suspected former exit of the tunnel in the Otay Mesa warehouse district within the U.S.

While there were no arrests, drug seizures or confirmed exit point in the United States, the length — more than 14 football fields — stunned authorities. It went under several warehouses in the Otay Mesa area, where sophisticated tunnels have typically ended, and extended into open fields.

U.S. authorities say they are confident that the tunnel exited in San Diego at one time, based on its trajectory.

LeNoir, a veteran on a multiagency task force of tunnel investigators known as “tunnel rats,” said he made his way through about 50 feet of sugar sacks blocking the tunnel but couldn’t go any farther.

An incomplete offshoot of the tunnel that extended more than 3,500 feet suggested to authorities that smugglers had plugged an initial exit point and were building another.

The suspected previous exit “became unsustainable for whatever reason, so they built a spur,” Border Patrol spokesman Jeff Stephenson said.

By federal law, U.S. authorities must fill the U.S. side of tunnels with concrete after they are discovered.

“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 in San Diego.

Authorities have found 15 sophisticated tunnels on California’s border with Mexico since 2006, with hallmarks including lighting, ventilation, railway tracks and hydraulic lifts.

The tunnels are concentrated in an area where clay-like soil is conducive to digging and industrial warehouses provide cover.

The latest tunnel was discovered by Mexican authorities in a Tijuana parking lot for cargo trucks, next to the city’s airport.

U.S. authorities mapped its course, drilled a hole on the U.S. side and lowered a camera to determine where to start making their way through.

“It was endless,” LeNoir said.

Authorities didn’t say who they believe was behind the tunnel but the area has been a stronghold of Mexico’s Sinaloa cartel. The cartel’s longtime leader, Joaquin “El Chapo” Guzman, was sentenced to life in U.S. prison in July.