Police discovered the tunnel, equipped with a hydraulic entry, an elevator and electric rail cars, on Tuesday [Reuters]

Mexican authorities have unearthed a 600-metre-long drug smuggling tunnel beneath the US border, the second such find in the same area this month.

The 15ft-deep tunnel found on Tuesday was similar to another tunnel found on November 16, and contained both lighting and transport carts, indicating the growing technical sophistication of the country's powerful drug cartels.

"The tunnel was located after we were informed of it by US authorities," who had found the other end of the passage in nearby San Ysidro, California, Ricardo Garduno, Tijuana's police chief, told the AFP news agency.

In Mexico, security forces who took part in the raid said three tonnes of marijuana were uncovered inside the tunnel.

But US authorities said in a statement hours later that 32 tonnes of marijuana, worth $65m, were confiscated in the operation.

The US Immigration and Customs Enforcement service described the find as a major victory over Mexican drug cartels responsible for vast trafficking across the border into the US.

Six suspects held

The San Diego Tunnel Task Force, on the US side of the border, said that six suspects had been arrested in the US following a six-month investigation.

The entrance to the tunnel on the Mexican side was found about 100 metres from the local headquarters of the federal police, a military air base and a customs office, according to an AFP reporter.

US authorities announced on November 16 the discovery of a "major" drug-smuggling tunnel under the border, running up to 400m long, equipped with electricity supplies and a ventilation system.

Some 45,000 people have been killed since 2006, when Mexico launched a major military crackdown against the powerful drug cartels that have terrorised border communities as they have battled over lucrative smuggling routes.

For three years, investigators have been finding highly sophisticated tunnels shortly before the winter holidays in what officials speculate is an attempt by drug smugglers to take advantage of Mexico's autumn marijuana harvest.

"You have an organisation that knows that they're going to have x amount of inventory at a certain time of the year, similar to legitimate businesses who monitor their inventory, and they want to find the most effective and efficient means to distribute that inventory and sell it as they can," Derek Benner, a special agent of the US Immigration and Customs Enforcement in Otay Mesa, California, said.

More than 70 passages have been found on the border since October 2008, surpassing the number of discoveries in the previous six years.

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

Many tunnels are clustered around San Diego, California's Imperial Valley and Nogales, Arizona. California is popular because its clay-like soil is easy to dig, whereas 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.

Source: Al Jazeeera and agencies