It was late February when a friend and I drove across the US border, from California to Mexico, on a sunny Saturday morning. The landscape was familiar: barbed wire fencing, and high intensity lights line the freeway as you approach the border patrol stops. But coming from the US, you breeze across to Tijuana with your foot barely touching the brake.

We did some shopping, taking advantage of bargain prices on outdoor furniture, ceramic tiles and prescription medicines. By afternoon we were prepared to head home. But of course the trip back to the US is different.

Cars wait in lines for upwards of an hour as border agents take their time looking at them and asking questions of drivers and passengers.

As our car, a late model Toyota convertible, approached the patrol booth, I removed my sunglasses. It is best to make eye contact with an agent whose gaze seems certain to discover your innermost thoughts.

"How you ladies doin'? Whatcha bringing back?" he asked casually. "Oh some toys, tiles, stuff like that," I said.

"Got any guys hidden back there?" he laughed, pointing to the car boot. "Not today!" I said, chuckling on cue. We were waved through.

It was difficult not to think of how easy it would have been to smuggle a few illegal guys into the US. In fact, many people, mostly Americans, have been unable to resist the temptation.

Some 43,000 cars pass through this same border crossing, called San Ysidro, each day. It is the busiest port of entry in the US, according to the Immigration and Naturalization Service. Stepped-up patrols and high-tech surveillance along the land border have resulted in fewer people trying to get across to the US by foot using this route. Instead, smugglers are increasingly bringing in people hidden in specially equipped cars and vans.

Mexicans, Central Americans, Chinese and even Iraqis are hidden in boots, under false floors or next to engines. Last year, some 340 people drowned or died of exposure trying to get into the US.

At this port of entry, there have been no deaths yet, but many aliens have been injured. In December, a man and a woman were discovered by an immigration dog as they lay in side-by-side gas tanks under the bed of a pick-up truck. They were breathing through respirator masks linked by hose to scuba tanks in the truck bed. The couple had paid $6,000 (£5,085) to the smuggler for the 40-minute drive.

Total arrests at this port and five others along the Mexican border went from a low of 48,882 in 1997 to 84,276 last year.

The going pay rate for drivers recruited to smuggle aliens is about $1,500 (£1,271).

For Americans, mostly young people who find it a way to make some quick money, the penalties are light. Generally, they are briefly detained while their names are entered into an INS computer registry of offenders.

Their human cargo, however, is sent back to Mexico and may forfeit any right to ever enter the US legally. Illegal immigration is a problem throughout the world and certainly not limited to the US. In fact, studies show that destinations other than the US, mainly Europe, are now far more popular. California, for example, once the number one destination in the US, no longer has the appeal it once did. An oversupply of day labour and frequently low wages are driving immigration elsewhere.

"California is no longer the promised land that it was for immigrants," said Dowell Myres, a demographer and urban planner, speaking to the Los Angeles Times.

The reasons for illegal immigration to the US are still primarily economic.

Thousands of Chinese, mainly from Guangdong and Fujian province, continue to flood into New York. Last year, a New York lawyer was indicted for smuggling Chinese via ships, charging them $40,000 (£34,000) to $50,000 (£42,372) each, and keeping them in indentured servitude until the fees were paid.

The living conditions of many Chinese illegals are dire, but even the paltry wages of $1,000 (£847) a month they earn, mostly in restaurants, are better than the $50 (£42) a month they get at home.

For others, political asylum remains a desperate motivation. Last September, 200 Iraqi Christians holed up in a Tijuana motel, seeking passage into the US. They had reportedly paid smugglers $10,000 (£8,474) each. Since the 1991 Gulf war, the US has granted asylum to 3,124 Iraqis, or 85 per cent of those who have applied. By contrast, only one in eight Chinese applications are approved.

But as illegal immigration becomes more difficult and complex, a vast network of smugglers is emerging to turn a profit. Last year, a prominent United Nations official warned that trafficking in human beings is the fastest growing business in organised crime.

Pino Arlacchi, an Italian sociologist who worked closely on fighting the Mafia before becoming director general of the United Nations Office for Drug Control and Crime Prevention, told the New York Times that the tragic deaths of 58 Chinese migrants trying to reach Britain last year was just the tip of the iceberg, and one of several recent accidents that show the magnitude and seriousness of the problem.

Human trafficking "is the fastest growing criminal market in the world because of the number of people who are involved, the scale of profits being generated for criminal organisations - and because of its multi-fold nature," said Arlacchi, now a UN under-secretary general. "We don't have just sexual exploitation. We don't have just economic slavery, which includes two things, forced labour and debt enslavement. We have also a lot of exploitation of migrants. And we have classic slavery. "If you put all this together under the same concept, you get the biggest violation of human rights in the world," he said.

Arlacchi is proposing that anti-slavery laws be reintroduced where they have lapsed or been eliminated. He is also considering recommending temporary residence for would-be immigrants who cooperate with authorities in identifying criminals who are trafficking in people, according to the New York Times.

"This a measure that in some countries, like Italy and Austria, is showing very important results in understanding the nature of networks - how the victims are attracted, how they are recruited and the way they are exploited," he said.

"But first of all, we really have to understand the size of the phenomenon," Arlacchi added. "It is a sensitive political issue. No country likes to admit to having sexual exploitation and human trafficking. So we have to be very careful how we deal with the source countries."