SAN DIEGO -- Authorities were removing an estimated 2 tons of marijuana from a "massive" cross-border tunnel that began near the Tijuana airport and ended near an apparently vacant industrial building on the U.S. side, officials said.

Authorities on Wednesday located the U.S. exit to the tunnel, which began inside a warehouse near the airport with a cement shaft about 10 feet wide and 7 feet long. The shaft dropped about 75 feet to the tunnel, which was armslength wide and high enough for an adult to stand inside.

The tunnel floor was cement, and lights ran down the side of one of the hard soil walls.

Mexican authorities allowed reporters and photographers in the tunnel late Wednesday night. Near the entrance, authorities were seen weighing what appeared to be marijuana. Several hundred packages wrapped with brown packing tape were stacked about 5 feet high.

The discovery of the exit prompted a criminal investigation by the U.S. Attorney's office in San Diego, said Lauren Mack, a spokeswoman for U.S. Immigration and Customs Enforcement.

Mack described the passageway as longer than most of the 21 cross-border tunnels discovered since authorities began keeping track after the Sept. 11, 2001 attacks.

"It's massive," she said.

Mexican investigators found the entrance to the tunnel Tuesday in the warehouse about 100 yards south of the border. The tunnel was equipped with a pulley system.

Also Wednesday, U.S. and Mexican authorities found an unfinished tunnel when a U.S. Border Patrol vehicle struck a sinkhole early Wednesday near the San Ysidro border crossing, which links Tijuana and San Diego. It was about two feet underground and extended about 30 feet into the United States, near a storm drain.

"It was very, very small and extremely primitive," Mack said.

The unfinished tunnel began just south of the border in Tijuana in a vacant lot, said Jose Marquez Padilla, a Mexican Customs director. It was about three feet wide.

Last week, Border Patrol agents discovered a 35-foot-long tunnel beneath the U.S.-Mexico border in after it caved in and the asphalt roadway above it collapsed. U.S. authorities said the tunnel ended in a patch of vacant land near the San Ysidro port of entry in San Diego. Three other tunnels have been discovered this month in the Tijuana-San Diego area.

CLICK FOR BUST IMAGES