SAN DIEGO (AP) -- With tougher drug enforcement above ground, authorities say traffickers along the U.S.-Mexican border were forced to dig deep below ground instead.

Inside a five-foot-wide tunnel, with just enough room for an adult to stand, authorities say they discovered two tons of marijuana this week, and what they believe was a passageway for drug trade.

The 2,400-foot long tunnel is lengthier than most of the 21 cross-border tunnels that have been discovered since authorities began keeping track after the Sept. 11 attacks, U.S. Immigration and Customs Enforcement officials said.

"It was like being in a cavern or a cave," said Michael Unzueta, customs special agent in charge in San Diego.

The tunnel had a pulley system on the Mexican side, which began near the Tijuana airport, and ended in a warehouse on the U.S. side, authorities said. Inside, it had a cement floor and lights mounted on one of the hard soil walls.

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