NOBODY paid much attention when a large white van with Arizona numberplates pulled into a parking space in the shade of the 15ft-high sheet-metal wall that separates Mexico from the United States in the divided border town of Nogales.

A few yards away US border guards monitored the steady flow of pedestrians through a heavily fortified gate in the wall. There was no reason to be suspicious of a van legally parked on a metered space directly beneath a pylon of American surveillance cameras.

Yet inside the van Mexican gangsters were preparing one of the most brazen smuggling ploys ever recorded along this notoriously leaky stretch of Arizona’s border with Mexico. The back of the van had a hole in its floor. The gangsters had acetylene blowtorches and tools to dig a hole in the street.

Using passing traffic to conceal the noise of their digging, the Mexicans broke the surface of the road beneath them and connected with a tunnel that their co-conspirators had dug under the border wall from a house close by on the Mexican side.

For at least two weeks, according to US federal agents, Mexicans crawled down the tunnel to pass drugs into the van. Illegal immigrants may have followed. Each time the van filled up, the entrance to the tunnel was covered with a metal plate and resealed with a thin layer of tar, easily removed the next time the van returned.

It took a tip from a paid informant to alert the US authorities to the smuggling under their noses. All that is left of the tunnel on the US side today is a tar smudge covering the hole, which the Americans filled with cement.

Mexico’s drug and people smugglers have long been renowned for the creativity of their cross-border ruses. Border agents still talk about Enrique Aguilar Canchola, who was found a few years ago disguised as a car seat — he had been covered in a seated position with stuffing and material that made him almost unrecognisable.

Yet despite the four-pronged efforts of US border, customs, drug and immigration agencies, low-tech cunning continues to haunt America’s increasingly expensive efforts to stem the tide of illegal traffic across the Mexican border.

Of the 1m arrests of immigrants — known locally as “wets” or “wetbacks” — by the US border patrol last year, more than 600,000 were made in the rugged desert of Arizona. Nobody can tell how many passed through tunnels or how many escaped undetected, but American agents are under increasing pressure to find new ways of detecting movement underground.

“There are so many tunnels under Nogales, we’re just waiting for the whole place to cave in,” said special agent George Gibson as he drove me around town pointing out houses, sheds, manhole covers and even a former church that had been used to conceal tunnel exits.

Mexican tunnellers are known to have been active along the border for years, but the issue has acquired a new urgency in Washington after the discovery in January of a half-mile-long tunnel built 80ft under the border fence at Otay Mesa, 15 miles southeast of San Diego.

Both the Mexican and American entrances to the tunnel were concealed in vacant warehouses on either sides of the border. It was tall enough for a man to walk through and was equipped with lighting cables and water pumps.

The tunnel must have taken a year to build and cost up to $1m. More than a ton of marijuana was seized when the US warehouse was raided in January. It may also have enabled thousands of immigrants to pour into America without being detected.

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