SAN DIEGO -- The investigation into the largest marijuana bust at a cross-border tunnel followed a familiar timeline. It began in May and ended in November.

The secret passage linking warehouses in San Diego and Tijuana -- equipped with a hydraulic lift, electric rail carts and a wooden staircase -- highlights an emerging seasonal trend.

For three years, authorities have found sophisticated tunnels on the U.S.-Mexico border shortly before the winter holidays in what officials speculate is an attempt by drug smugglers to take advantage of Mexico's fall marijuana harvest.

Two weeks ago, authorities seized 17 tons of marijuana in connection with a tunnel linking warehouses in San Diego and Tijuana. Authorities began investigating that passage in June, according to court filings.

Tuesday's find netted more than 32 tons of marijuana -- nearly 17 tons at a warehouse in San Diego's Otay Mesa area, about 11 tons inside a truck in the Los Angeles area and 4 tons in Mexico.

According to the U.S. Drug Enforcement Administration, it ranks as the second-largest pot bust in U.S. history if the drugs found on the Mexican side of the tunnel are counted, and the third-largest without the Mexican stash.

As U.S. authorities heighten enforcement on land, tunnels have become an increasingly common way to smuggle enormous loads of marijuana. More than 70 passages have been found on the border since October 2008, surpassing the number of discoveries in the previous six years

Raids last November on two tunnels linking San Diego and Tijuana netted a combined 52 tons of marijuana on both sides of the border.

In early December 2009, authorities found an incomplete tunnel that stretched nearly 900 feet into San Diego from Tijuana, equipped with an elevator at the Mexican entrance.

Authorities said central Mexico's marijuana harvest in early October presents drug cartels with a familiar challenge for any farmer -- how to quickly get products to consumers.

"It's a significant amount of inventory that the cartels need to move, and they need to move it in the most expeditious and efficient way," said Derek Benner, U.S. Immigration and Customs Enforcement's special agent in charge of investigations in San Diego.

"It's like any other business. You've got a pile of inventory that you need to get moving and generate profits."

Authorities said they don't know whether cartels are building the tunnels in time for the winter holidays, or if that's when authorities just happen to find them.