Tis the season for finding cross-border drug tunnels.

The latest secret passage -- equipped with a hydraulic lift, electric rail carts and a wooden staircase -- was discovered Tuesday on the U.S.-Mexico border, highlighting an emerging seasonal trend. For three years, authorities have found sophisticated tunnels shortly before the winter holidays in what officials speculate is an attempt by drug smugglers to take advantage of Mexico's fall marijuana harvest.

The discovery of the 600-yard tunnel resulted in seizures of 32 tons of marijuana, one of the largest pot busts in U.S. history. It linked warehouses in San Diego and Tijuana and was equipped with lighting and ventilation. Wooden planks lined the floor about 40 feet underground.

"This is an incredibly efficient tunnel designed to move a lot of narcotics," said Derek Benner, U.S. Immigration and Customs Enforcement's special agent in charge of investigations in San Diego said Wednesday.

Authorities recovered nearly 17 tons of marijuana at the warehouse in San Diego's Otay Mesa area, nearly 12 tons inside a truck in Los Angeles and about 4 tons in Mexico. Six people were charged in federal court in Southern California with conspiracy to distribute marijuana.

As U.S. authorities heighten enforcement on land, tunnels have become a major tack 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

Two weeks ago, authorities seized 17 tons of marijuana in connection with a tunnel that linked warehouses in San Diego and Tijuana.

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.

"If they can't cross the border above ground, they attempt to tunnel underneath it," said Laura Duffy, the U.S. attorney in San Diego.

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

William Sherman, the U.S. Drug Enforcement Administration's acting special agent in charge in San Diego, said drug traffickers also may go on a pre-Christmas smuggling push to give themselves a "little bit of hiatus" over the holidays to visit family in Mexico. DEA wiretaps tend to go quiet during the holidays, he said.

It's unclear whether cartels are building the tunnels in time for the winter holidays or if that's when authorities just happen to find them. Some U.S. authorities are inclined to think the cartels are timing construction for the fall harvest, based on their belief that this year's two major finds in San Diego and one last year in San Diego were discovered shortly after they were completed. Heightened activity around building and operating the tunnels drew suspicion and exposed smugglers to getting caught.

It takes roughly six months to a year to build a tunnel, authorities say. Workers use shovels and pickaxes to slowly dig through the soil, sleeping in the warehouse until the job is done. Sometimes they use pneumatic tools.

Many tunnels are clustered around San Diego, California's Imperial Valley and Nogales, Ariz. California is popular because its clay-like soil is easy to dig. In Nogales, smugglers tap into vast underground drainage canals.

San Diego's Otay Mesa area has the added draw that there are plenty of nondescript warehouses on both sides of the border to conceal trucks getting loaded with drugs. Its streets hum with semitrailers by day and fall silent on nights and weekends.

After last November's twin finds, U.S. authorities launched a campaign to alert Otay Mesa warehouse landlords to warning signs. Landlords were told to look for construction equipment, piles of dirt, sounds of jackhammers and the scent of unburned marijuana.

Tuesday's seizure rank as the second-largest pot bust in U.S. history if the drugs found on the Mexican side of the tunnel are counted, according to the DEA. It is the largest seizure ever linked to a tunnel.

The tunnel passage was about 4 feet wide and 4 feet high. It featured a wooden staircase at the U.S. entrance, located inside a large, white building with a long line of trucking docks.

The Mexican entry point was on the same block as a federal police office and featured a hydraulic lift at the tunnel entrance that dropped about 30 feet. Neither the U.S. nor the Mexican warehouse had exterior signs.

The Mexican warehouse sits next to a runway at Tijuana's main airport. Its carpeted floors were found littered with garbage and dirty linen. The kitchen was stocked with tortillas and oranges, with a window painted black.

No arrests were made in Mexico in the latest find. U.S. authorities linked last November's twin discoveries to Mexico's Sinaloa cartel, headed by Joaquin "El Chapo" Guzman, that country's most-wanted drug lord. No link has been established to the Tuesday bust.

Sherman said the Sinaloa cartel's growing presence in Tijuana may help explain the massive loads.

"Sinaloa has traditionally run a lot of marijuana through other ports, and I think now that they have more control of this port that's certainly accounting for the large quantities we've been getting here in the last two years," he said.