By sheer good fortune, Hong Kong, the world's busiest container port set in a stretch of the most crowded waters, has so far managed to avoid the marine catastrophes which have happened on other shores. There have been no massive oil slicks, no major collisions. None of the accidents which make international headlines, and turn entire coastlines into disaster areas. But it takes a relatively small incident, like the sinking of the Guan Hang off Tuen Mun this week, to remind us of how vulnerable Hong Kong is to a crisis on the sea. In 1996, 450,000 craft passed through these waters, patrolled by the 20 vessels of the Marine Department. Most of the department's energies are concentrated on checking ocean-going traffic, which last year amounted to 219,221 ships. But it is the small boats that ply back and forth between the mainland and the SAR which regularly overload. Taking on cargo in the flat waters of the Pearl River Delta they seem safe enough, until they arrive in the crowded seaways of Hong Kong, where the surface is choppy with the wash of vast container ships and hundreds of other craft. Already low in the water, they can be spotted from any ferry battered by the wake of passing ships, with their decks awash and so perilously close to sinking that the wonder is that so many of them actually manage to hobble in to port. Had it not been for the cargo of ammonium chloride lost in the seas off Butterfly Bay, the sinking of the Guan Hang would probably have been simply one more incident in a catalogue of small events which take place off Hong Kong at regular, unnoticed intervals. Pollution pours into our waters in small but ceaseless measure. Ships cleaning out their tanks merely add to the effluent and industrial waste which is hourly dumped into the sea in the belief that toxins can be absorbed indefinitely. The concern over the Marine Department's slow response to the ammonium chloride container is a touch ironic under such circumstances, but if it raises public consciousness to the ever-present danger, it will have done the SAR a favour. The local fishing industry is at a virtual standstill. The SAR's unique pink dolphin is dying out. Red tides and blackened beaches spell out a doomsday message, but no one seems to take any notice until a lost container of harmful chemicals makes headline news. It is the incidents which pass unnoticed which should worry us most of all.