How many harbours play host to everything from seahorses and frogfish to whales and dolphins The most exotic wonders of the underwater world are just a jump and a swim away for Sydneysiders, says Paul Adams - only most of them dont know it

My torch beam sweeps through the black water, throwing a pool of light onto a rock wall blotched with sponges of orange, red, yellow and pink. As I move closer, I can see that the sponges in turn play host to many other creatures.
To my right, a pair of nudibranchs, sea-slugs no more than an inch long, graze lazily on the sponge. The name slug demeans these creatures, because their gaudy patterns of stripes and spots are often more colourful, as in this case, than the brightest of parrots.
A cuttlefish the size of my hand hovers near the wall, regarding me with sleepy-looking eyes. Like the top of a lemon meringue pie, its skin is peaked, mottled brown and white, but in a split-second it changes both texture and colour to become a smooth and uniform brown.
Less than a metre to the left, a bright red scorpion cod nestles among folds of orange sponge. It believes itself invisible, like many other masters of camouflage, and it is almost right.
Wherever my torch-beam sweeps, more secrets of this underwater garden are uncovered. Yet I know I could spend the whole dive kneeling in front of this single sponge-covered rock, and still not see all there is to see.
My buddy taps his wristwatch. An hour and a half have passed since we left the surface, and air is getting low. We ascend slowly, but from 7m it takes barely a minute to reach the surface.
We take our demand valves from our mouths, able to speak for the first time in 90 minutes. Below us is the beauty, the colour, the extraordinary excitement of the sea; to our left, the beach and houses of Camp Cove. Looking west, beneath the clear night sky, are the towering lights of downtown Sydney. What can I say Welcome to Sydney Harbour.
Fifty-five square kilometres in area, Sydney Harbour has 150 miles of convoluted foreshore and dozens of tiny bays and inlets. This beautiful expanse of water was once the valley of the Parramatta river, drowned by rising sea levels following the big thaw at the end of the last ice age.
Admired by all who see it and treated in Australia as a national icon, Sydney Harbour attracts more than its share of labels. It is a working harbour, plied constantly by the 27 ferries of the State Transit fleet, together with tourist boats, container ships, tankers, cruise liners and navy vessels. At the weekends, yachts and cruisers of all sizes criss-cross the blue water.
But for all the people who commute, sail and swim in the harbour, or aspire to own a piece of land with harbour frontage, how many ever wonder what is really down there In a city of 4 million people, underwater Sydney is a mystery to all but a few.
I migrated to Australia in 1992 from the UK, after spending some time in the Middle East. A keen scuba diver, I had grown fond of the outdoor lifestyle of camping and diving that we enjoyed on the shores of the Red Sea, but was less keen on the restrictions we had to face day to day.
A holiday in Sydney was what made up my mind to come to Oz. It was the best of both worlds. But even I was amazed when I submerged for the first few times in the waters of Sydney Harbour. The murky soup of plastic bags and dubious pollutants I had conjured up was just that - imagination.
Four years later, I am constantly surprised by the remarkable variety of life encouraged to live in Sydneys watery backyard by its diverse ecosystems. More than 581 species of fish have been recorded in the harbour, including many juvenile tropical species, carried south by the warm East Australian Current during the summer.
Popular spots for watching marine life include Camp Cove and Fairlight, which harbour rocky reefs, home for kelp, multi-hued sponges and sea-squirts. The adjoining sandflats are covered in seagrass and home to pipefish, close relatives of the seahorse. Resting on the sand are stingrays, electric rays, frogfish and the docile Port Jackson shark.
Octopuses are a common sight, crawling across the bottom, hiding under rocky ledges or holed up in shell middens in the sand - homes made from the remains of earlier meals. A relative, the tiny pin-striped dumpling squid, no bigger than a thumb, can often be found nestled in the sand with only its eyes poking out.
Exotic juvenile fish such as firefish can be found in Camp Cove each year, but they rarely make it through Sydneys chilly winter waters.
Clifton Gardens, a reserve on the shore of Chowder Bay, was once the site of a rowdy dancing pavilion during the 19th century. Today, Clifton Gardens after dark belongs to the night diver.
A wharf, supporting a steel-ringed shark net on the inside, forms a pool. These linked rings, similar to those favoured by conjurers, support a thick blanket of animal and plant life.
Speckled moray eels slither between the links, winding themselves around the large red and yellow sponges which festoon the net. Dozens of seahorses cling to the metal rings with their prehensile tails, though they are very hard to spot, blending perfectly with the seaweed draped on the net.
The wharfs pylons are shrouded in vivid sponges, host crabs, nudibranchs and pygmy leatherjackets - tiny fish that hang on to weed or sponge by their teeth.
There are many other popular sites in the clearer waters towards the entrance, most of them near beaches or reserves. Few things are as pleasant as surfacing from a dive to enjoy a picnic or barbecue with family or friends overlooking the harbour.
Further east, a boat is needed to dive the wonderful sponge gardens of Sydney Harbours north head. Beneath the cliffs at a depth of 20m or so, huge boulders are piled high, separated by narrow crevices and tunnels. Shoals of bullseyes crowd these holes, shying away from a divers torch beam.
Both inside and outside, massive encrusting sponges splash colour on the grey rocks, while mace-like sea tulips wave in the current. Shoals of yellowtail and sea-pike cruise along, flashes of silver in the blue. Bright red and yellow gorgonian fan corals spread perpendicular to the current, their polyps filtering plankton for food with the least effort. Tube sponges reach hand-like for the surface.
The water is often very clear here, and you can swim along the edge of the sand, exploring both the rocks and the sandy bottom. This holds its own surprises, such as large rays and the docile angel shark.
Outside the harbour, Sydneys marine world has yet more to offer. Sites such as Gordons Bay near Clovelly offer clear blue water, easy access and the chance to meet friendly blue grouper or see eagle rays fly across the sand. Offshore there are deep reefs and historic wrecks to explore. Boats are run frequently to these sites by Sydneys many dive stores and clubs.
When the first fleet sailed into Port Jackson in January 1788, its surgeon-general declared this beautiful harbour the finest in the world. Following European settlement, pollution increasingly took its toll, but these days deepwater outflows take domestic sewage far offshore, and industrial pollution of the harbour was outlawed in the early 1970s.
Each year, Clean Up Australia Day sees dozens of divers scouring the bottom of the harbour for old junk. Recently Sydneys Ryde Underwater Club removed 30 bags of bottles, cans, corrugated iron, rusty oil barrels and crockery from the shallows of picturesque Shark Island in the harbour.
The club made a point of leaving any bottles which had been taken over by marine life, as they had become part of the ecosystem. It was encouraging to see that many of the bottles collected were decades old. Perhaps fewer are thrown in nowadays - I like to think so.
Parts of north Sydney Harbour have been declared an aquatic reserve, and the whole harbour is now an intertidal protected area, effectively banning the collection of shellfish and other invertebrates. Dolphins and whales have been spotted recently, for the first time in years, and oysters have returned to harbour shores in large numbers.
The ecosystem remains fragile. Alarms were raised when industrial glue was discharged into the harbour illegally, early in 1996. That November, toxic algae, thought to derive from the illegally dumped ballast water of a foreign ship, killed many fish and threatens to become a recurring problem.
And only last August, 300,000 litres of light crude oil was spilled into the harbour by the tanker Laura DAmato. Most of this was, luckily, contained by booms at Shells terminal - and the spill did not prevent southern right whales entering the harbour less than a month later.
If the ocean is the silent world, Sydney Harbour is its silent suburb. Resident divers are doubly fortunate to have a beautiful harbour and such magnificent marine life in their backyard. No traffic noise, just bubbles and the bizarre creatures of the sea for company. We aim to keep it that way.

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