Discovery Bay Marine National Park (2,770 ha) is Victoria's marine gateway to the Great Australian Bight and the immense expanse of the Southern Ocean.  The cliffs of Cape Bridgewater to the south and the white sand dunes of Discovery Bay to the north frame the Discovery Bay Marine National Park.   

The coastline is formed from the remains of a massive volcano that has spewed forth lava over the last million years, cooling into hard basalt rock. The Marine National Park is part of the largest coastal basalt formation in western Victoria and experiences one of the highest wave energy environments in the State. It has rocky habitats of complex forms interspersed by plains of sand winnowed into symmetrical ridges by the endless movement of the water. 

The seafloor of the area reflects this dynamic history.  Intertidal and near-shore reefs formed from basalt border the park.  In deep water (30-60 metres) within the park, there are low profile reefs formed from ancient shorelines or dunes when the sea levels were much lower than today sometimes capped by volcanic basalt.  These reefs are covered in rich sponge 'gardens', composed of thick growths of sessile invertebrates including sponges, ascidians, bryozoans and gorgonians.   

The rich sea-life is derived in part from cold nutrient-rich water associated with the Bonney Upwelling sucked up towards the surface by offshore winds and currents during summer.  This water encourages the growth of microscopic plants and animals, providing a feast for fish, whales, penguins, and seafloor life such as sea-fans, sea-mosses and hydroids, sponges, and sea-squirts. 

The connection to the Great Australian Bight and the Southern Ocean is highlighted by the regular visits of Southern Right and Humpback Whales that migrate north from Antarctic waters in winter to breed, and Australian Fur Seals foraging from the nearby colony on Cape Bridgewater. Blue whales, the largest of all creatures to have ever lived, also regularly visit these waters during summer.