SAN DIEGO -- U.S. Border Patrol agents Friday discovered a marijuana-filled truck parked over a covered hole in the ground that concealed a "sophisticated tunnel" for transporting drugs under the border.

Smugglers would apparently drag bundles of marijuana from the Mexican side of the border through the tunnel, coming up just under the truck inside the U.S. hidden from view by the vehicle, they would hand the illegal drugs to an accomplice in the truck through a hole cut in the truck's bottom, according to a Border Patrol official.

Agents were on routine patrol when they discovered the "U-Haul-type truck" containing more than 3,000 pounds of marijuana in the 24-hour parking lot immediately adjacent to the San Ysidro Port of Entry, Border Patrol spokesperson Ben Bauman said.

Then, while they were "cleaning up the scene," the agents noticed a "concrete slot" cut into the ground where the truck had been parked, according to Bauman.

Beneath the slot was a "sophisticated tunnel" with lights, plastic sheeting and bracing materials to keep it from caving in, Bauman said.

"It looks like they would crawl through the tunnel with a bundle of marijuana, go to the very end where they could open this concrete slot, slide the bundle through, and the person in the vehicle would pick it up through a hole in the vehicle," Bauman said.

It wasn't clear what triggered the agents' interest in the truck. Border Patrol and Drug Enforcement Administration officials were at the scene continuing their investigation, Bauman said. No further details were immediately available.