Port Melbourne c.1913-14, (detail) Ambrose Patterson, oil on board, private collection. Port Phillip Bay, known by that name for less than 180 years, is not the sort of place to boast brassily of its charms, which are subtle. Spectacular beaches, towering promontories and majestic views are hardly abundant around its 1900 square kilometres. Yet it is not just a great, boring, grey puddle, as Marris, an aquatic scientist and 2002 young Australian of the year (environment), is about to reveal at Jawbone. As we plunge into this lush sanctuary, nestled between the flaming plumes of the Altona Mobil refinery stacks and the glitzy faux-mansions of Williamstown's Rifle Range estate, a vibrant netherworld appears. Within minutes, we've seen a sparsely spotted stingaree, followed during the next two hours by several schools of zebra fish, two tubby puffer fish, a smooth stingray and barbless fiddler rays, one of which Marris cradles in her hands. That's not to forget different coloured corals, fields of waving sea grass and drifts of bright-green sea lettuce, as well as white mangroves with exposed roots, various species of sea snail, a sea urchin and an array of starfish, including the enormous 11-armed seastar and the caramel-coloured biscuit star.

The emigrant, 185?, (detail) artist unknown, 1850s lithograph (hand-coloured), National Library of Australia, Canberra, from Sea of Dreams. Marris' latest book, Melbourne Down Under: The Jew-els of Port Phillip Bay, contains incredible images. When she showed these photos to potential sponsors or publishers, some scoffed. Even a few old salties refused to believe such creatures — tropical coloured nudibranchs, 25-centimetre seahorses or iridescent sea slugs — existed in the bay. Surely, these pictures were taken somewhere much more exotic and equatorial, not in our bay. Some shots do look as if they were taken on the Great Barrier Reef, but all this beautiful flora and fauna was photographed, and can be found right under our noses or, more precisely, under our snorkels. Slumbering sea Mentone, 1887, (detail) Tom Roberts, oil on canvas, NGV, from Sea of Dreams. It is strange to think the bay hasn't always existed. In truth, it's only been there for a blip in geological time and the shore-dwelling indigenous inhabitants — the Wathaurung, Woiwurung and Bunurong, part of the Kulin nation — have cultural memories of when bay-area volcanoes erupted.

Those people may have been in the area for 40,000 years or more but the bay wasn't there 18,000 years ago during the Pleistocene ice age. Then, it was just a huge, frosty plain criss-crossed by mighty rivers, which eventually became the Yarra and the Maribyrnong. Every time I go in the water, I see something different. About 8500 years ago, Port Phillip Bay as we know it began to form as sea levels rose. So much ice melted that 3000 years later the water level was two metres higher than now — there were lapping waves where Melbourne's central business district now sprawls. Latest research suggests the bay may have dried up again because of a blockage at the Heads that was eventually breached about 1000 years ago, with the waters tumbling back in. The tribal people watched it all and, as historian Gary Presland writes in Aboriginal Melbourne: The Lost Land of the Kulin Peo-ple (1994), the tribes adapted slowly to the dramatic environmental changes. But nothing could prepare them for the rapid changes that followed European settlement, when the first explorers sailed through the Heads, bringing their lust for land, wealth, industry and colonisation.

As we stand on the beaches of Brighton, St Kilda or Altona, at the Heads, or the rise of Mount Eliza, or at the Docklands, it is hard to imagine the bay's physical boundaries having once been so different — or that we ought to be preparing for it to alter again. Recent New Scient-ist and Scientific Amer-ican reports have warned of huge rises in sea levels in the wake of global warming. New Scientist says studies of sea level and temperatures in the past million years suggest that a rise of only one degree in the global mean temperature could lead to a 20-metre rise in sea levels. As cities such as San Francisco and New York develop policies to deal with the threat of inundation, glaciologists predict sea levels could rise by about one metre by 2100. In the long term, if the ice in Greenland and Antarctica melts, it could result in a 60-metre rise in sea levels, they say. Each metre that the sea-level rises is likely to result in between 50 and 100 metres of coastal land being inundated. For now, Port Phillip Bay seems pretty much as it was when John Batman sailed in from Tasmania to buy the beautiful green grasslands from the local tribes. Migrants from Britain in the early 19th century would have experienced the same sights, too, if their ships were lucky enough to survive the perils of Bass Strait. Rodney James, a senior curator at the Mornington Peninsula Regional Gallery, is intimately acquainted with the bay. While researching a new show, Sea of Dreams: the Lure of Port Phillip Bay, James reflected on paintings that cast the bay in its true majesty.

There are summer-scented Ricketts Point and Sorrento paintings (1888-90) by Charles Conder, glorious Mentone views (1887-88) by Tom Roberts and works by Eugene von Guerard, Arthur Streeton, S. T. Gill and an especially atmospheric painting by Emma Minnie Boyd of the Esplanade from St Kilda Pier (1896). James, who has spent most of his life on the peninsula, says when he lived on the surf side, he thought the bay "a little bit too still, that nothing happened there". Since moving to Safety Beach, near Dromana, he has revelled in what he describes as more a huge inland sea than a bay. "When you live in proximity to the bay, and you see the nuances of it, it is a really interesting place that is constantly changing," he says. "You can see the weather as it comes towards you and the aftermath of storms, where the sun pierces through rain-sodden clouds. "You also get a feel, too, for how important it is as the gateway to Melbourne and Victoria. You see these huge tankers coming towards you, making a bee-line . . . and then they turn and go towards the channel back to the port.

"It is a place that has a lot of natural beauty . . . but also it is a working place, a place of industry." As he researched the exhibition, he repeatedly found comparisons with Sydney Harbour — and many commentators had greater appreciation for the bay than he expected. "Even in the 1880s, in The Argus, someone was writing comparing the two and saying we don't have the striking cliffs or the stunning colours but, in the end, there is a lovely familiarity that makes it our bay." Diary entries by immigrants describe long, arduous voyages across the world, a frightening passage into the Heads, then elation when they approached the shore and saw the park-like appearance of the land and its rich flora and fauna. For many new arrivals, emigration was both a hopeful experience and a heartbreaking one, director of the Mornington gallery, Jane Alexander, says. The bay was their first experience of a new life — the gateway to riches, especially during the gold rush. But these early immigrants were unlikely to see many of their loved ones who stayed behind. Her own family history, she discovered, is full of such stories.

The Mornington exhibition records a wealth of history about the bay: its first known exploration in 1802; the short-lived settlement at Sorrento in 1803; the threat of invasion by world powers; the emergence of Melbourne from cultural backwater to economic and cultural powerhouse of newfound wealth; the massive deforestation and environmental changes that followed the new settlements; and, of course, the destructive effect on tribes that had lived undisturbed in the region for thousands of years. More recent disturbances have included the dredging of the bay to deepen shipping channels, and the increasing pressure of pollution on the catchment area and marine life. For Sheree Marris, these are pressing questions. She cares deeply about the bay and her new book is part of a wider campaign to protect this fragile environment. She will donate 15 per cent of profits to community groups to encourage marine conservation. "About 85 per cent of marine life found here in the southern shores is found nowhere else in the world," Marris says. "Every time I go in the water, I see something different. I get so excited. It is my home away from home — the only place that I can relax and chill out and not think about anything. "The funny thing is that [while growing up], I never knew exactly how awesome it was. I used to want to be a marine mammalogist, travel around the world and study whales and dolphins."

But during a study group tour for university, she turned over a rock and saw a brightly coloured, blue and yellow nudibranch. "I was blown away to find something so incredibly delicate and beautiful without having to go up to the north, and without even having to go snorkelling. Imagine what else I might see. It became, for me, about finding out what else we have on our front doorstep. I thought, 'I don't need to go to the Great Barrier Reef.' " Marris is perfectly happy here in Port Phillip Bay — at home with its quietly revealed treasures. ■Sea of Dreams is at Mornington Peninsula Regional Gallery until February 19, mprg.mornpen.vic.gov.au ■Melbourne Down Under by Sheree Marris, $39.95, shereemarris.com

■The Piers Festival is on January 28, multiculturalarts.com.au Big bay ideas ■THE ECO ATOLL Architect John Macdonald, director of DesignInc Melbourne, formed a consortium of town planners, real estate agents and architects several years ago with the idea of building an Eco Atoll on top of the spoils of rock and soil from the recent dredging of the bay. The idea was to turn the dredged material into man-made islands with bird sanctuaries, dive sites and a marina. The diggings ended up elsewhere in the bay but Macdonald is still keen on the idea - lately he has been considering it an ''energy island'', 10 kilometres offshore with wind turbines, solar photovoltaic panels and wildlife sanctuaries. ■TOURIST ISLANDS

Another idea is to establish two islands at Spoil Ground, about 15 kilometres from Williamstown, Werribee and Sandringham. One island, at about six hectares, would be modelled on Perth's car-free Rottnest Island with restaurants, a harbour, five-star and backpacker accommodation. The other would be a bird and fish sanctuary. The Mornington Peninsula group making the proposal estimates costs at about $60 million and says it would be good for tourism. ■COMMUTER FERRIES Passenger ferries once ran between Station Pier and Geelong, Portsea and other peninsula ports but these days the only one used by anyone other than tourists is the ferry crossing the Heads. As Melbourne's population has grown and public transport has failed to satisfy, there have been calls to re-establish ferries from places such as Geelong, the peninsula and Williamstown. ■FLOATING HOUSING With pressure to contain the spillage of Melbourne's vast suburbs, one idea has been to mimic a Japanese style of architecture known as ''metabolic'' housing - which would entail floating apartment blocks in the bay. Everyone would have a sea view but getting home on a stormy night might be challenging.

■A BRIDGE ACROSS THE HEADS Loading More than one visionary has suggested a bridge from Queenscliff to Portsea to give Melbourne the architectural icon it craves, a structure to rival the Sydney Harbour Bridge and form a magnificent gateway into the bay. The idea has been around since the 1950s and is believed to be easy to construct; critics have suggested a flying fox instead.