TECATE, Calif., Dec. 6  The tunnel opening cut into the floor of a shipping container here drops three levels, each accessible by ladders, first a metal one and then two others fashioned from wood pallets. The tunnel stretches 1,300 feet to the south, crossing the Mexican border some 50 feet below ground and proceeding to a sky-blue office building in sight of the steel-plated border fence.

Three or four feet wide and six feet high, the passageway is illuminated by compact fluorescent bulbs (wired to the Mexican side), supported by carefully placed wooden beams and kept dry by two pumps. The neatly squared walls, carved through solid rock, bear the signs of engineering skill and professional drilling tools.

Shrink-wrapped bundles of marijuana, nearly 14,000 pounds worth $5.6 million in street sales, were found in the shipping container and in a trailer next to it, making clear the tunnel’s purpose: to serve as another major smuggling corridor. Found Monday here in Tecate, it is the latest of 56 cross-border tunnels found in the Southwest since the onset of additional guards and fencing aboveground after Sept. 11, 2001.

“I’m never alarmed when they are found,” said a senior investigator with a task force of federal law enforcement agencies still combing the scene Tuesday. “I am alarmed we don’t find them enough.”