It doesn't make sense until you realize that you're looking across the Shen Zhen into Hong Kong territory. Running parallel to the river is a border defense system meant to keep the mainland Chinese out. A chain of sodium-vapor lamps and a high fence topped with razor ribbon cuts across lakes and wetlands that have become wildlife refuges by default.

The population of Shenzhen is 2.6 million. Thirteen years ago it was two thousand. The growth rate of Guandong Province, which includes the Pearl River Delta, is the highest in the world.

It would be a lot higher if not for the Second Border that separates the Special Economic Zone from the rest of China.

When you're leaving Shenzhen you simply cruise through a chute without stopping. When you recross the Second Border on your way back, it's a different story.

The highway broadens into a vast slab of pavement covered with fine red dust from the Pearl Delta's devastated hillsides.

You and all other passengers have to bail out and pick your way hazardously through traffic until you've reached the border station.

Here you are funneled through one of many parallel lanes and checked out by a man in a uniform. If you're a Westerner, they don't even bother to look at you. If you look Chinese, you may have problems. A non-Chinese passport will get you through, of course, unless it's a British passport from Hong Kong; since the PRC doesn't recognize the legitimacy of Hong Kong, such people have to get a special document that serves the function of a passport inside the PRC.

If you are mainland Chinese, you don't get through unless the government has given you permission to live in the Special Economic Zone. Generally, such permission is only given to the young and college-educated. So Shenzhen has its own corrugated shantytowns of illegal immigrants, sitting in plain sight next to major highways, in the occasional patch of land that hasn't been covered with high-rises or factories yet.