Hong Kong's suburban rail network ends at Lo Wu, a small, sweaty settlement that would be wholly unremarkable were it not the front half of the main valve between the planet's most populous country and the rest of the world. Every few minutes, a train wheezes to a halt and disgorges hundreds more passengers. Hop over a series of official hurdles, and you suddenly find yourself ejected into the middle of a seething city. From being the cosseted tourist a few minutes ago, you are transformed into an alien.
All the clues that you normally use to orient yourself are useless in this part of the Orient. Look for a landmark or a street name to get your bearings, and all you see is a scrabble of graceful but impenetrable Chinese characters. Even the sun shelters behind a layer of high-octane smog, denying you the chance of getting a directional fix. The local characters sipping tea in the cafes are used to wide-eyed backpackers carving a trail of bewilderment through Shenzhen, so you barely merit distraction from the synchronised pecking at snacks. Elevenses already, and you still have to cross China.
Yet all you are really trying to do by teatime is to clip a tiny corner of a huge country, a journey of no more than 100 miles. And to make life easy, this is the most prosperous and advanced part of China. Shenzhen City is the high-rise hub of a Special Economic Zone that borders Hong Kong and thrives on the same enterprise culture. Sooner or later, a besuited businessman will take pity on the confused tourist and steer you towards the right bus.
At about the point on the bus ride when you guess that the broad city street must finally dissolve into a country road through profoundly green fields - it accelerates into a motorway, speeding straight to Guangzhou. The route to Macau, though, slips off to the left and the town of Humen. You get tipped out of the bus into the care of another well-spoken entrepreneur, who quits his mobile phone for long enough to steer you in the direction of the town's official tourist attraction: the opium museum.
Compared with the attractions en route, the historic monument of Humen is something of a side-show. But as the clock on Britain's lease of the New Territories ticks towards its 1997 expiry, the site acquires poignancy. In 1839, an uprising against the British drug barons forced them to hand over a huge consignment of opium, which was burnt on this very riverbank. But four years later the British forced China to allow them to build a fort on the site, to help them re-establish the trade in opium that made rule from London so hard to shake off.
Any traces of Anglicisation were extinguished during Mao's rule, so again you must seek help to set you on the next stage of the journey. A single bus, it appears, will take you almost to the frontier of Macau.
Buses get a poor press compared with the praise heaped upon trains, but this one would be a contender for any collection of Great Bus Journeys of the World. Not for the vehicle itself, a rudimentary beast that had clearly done this thousands of times before. Nor for the roadside scenery, a pleasant but unexceptional collage of agriculture and activity. The thing that makes this an amazing journey is the crossing of the Pearl River.
The inevitable new bridge over this three-mile divide will put an end to a startling piece of maritime theatre. The road suddenly ends in a delta of slip roads, each threading up to a boarding ramp. A fleet of smoke-belching ferries, squat and ugly, perform the most graceful marine dance. They deftly side-step one another as they shuttle back and forth, each one pausing only long enough to roll off one cargo of buses and trucks and roll on the next. From the deck, make the most of this unexpected boat trip to survey the frenzied shuffle and admire the fine embroidery that the wakes create on the surface of the muddy Pearl.
The last leg of the bus ride whisks you down the far side of the estuary, the skyline climbing as you approach Macau: buildings rise in proportion to the proximity of capitalism. The bus terminates some way from the border, but the improbably bulky luggage of your fellow passengers marks them out as transit travellers. You follow the procession of stripy red/blue bags bulging with cheap exports to the frontier.
Departure from the People's Republic is smoother than arrival, allowing you to slalom rather than stutter past the bureaucracy. You emerge into a strangely familiar post-colonial cityscape, joyful to be a regular tourist once more. Never has a former Portuguese outpost felt so comforting.
You take the ferry back to Hong Kong.