Portsea Hole is a depression in the seafloor

Seabed

The seabed is the bottom of the ocean.- Ocean structure :Most of the oceans have a common structure, created by common physical phenomena, mainly from tectonic movement, and sediment from various sources...

 of Port Phillip

Port Phillip

Port Phillip Port Phillip Port Phillip (also commonly referred to as Port Phillip Bay or (locally) just The Bay, is a large bay in southern Victoria, Australia; it is the location of Melbourne. Geographically, the bay covers and the shore stretches roughly . Although it is extremely shallow for...

 near Portsea, Victoria

Portsea, Victoria

Portsea is a resort town located across Port Phillip from Melbourne, Victoria, Australia. Its Local Government Area is the Shire of Mornington Peninsula....

, Australia

Australia

Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

. It is one of six separate areas that comprise the Port Phillip Heads Marine National Park

Port Phillip Heads Marine National Park

The Port Phillip Heads Marine National Park comprises six separate sites, with a combined area of 35.8 km², located in the vicinity of the entrance to Port Phillip, between the Bellarine and Mornington Peninsulas, in Victoria, Australia...

 and is a popular site for divers

Recreational diving

Recreational diving or sport diving is a type of diving that uses SCUBA equipment for the purpose of leisure and enjoyment. In some diving circles, the term "recreational diving" is used in contradistinction to "technical diving", a more demanding aspect of the sport which requires greater levels...

.

The Portsea Hole is a remnant section of the drowned valley of the Yarra River

Yarra River

The Yarra River, originally Birrarung, is a river in east-central Victoria, Australia. The lower stretches of the river is where the city of Melbourne was established in 1835 and today Greater Melbourne dominates and influences the landscape of its lower reaches...

, descending sharply from the 12 m depth of the surrounding seabed to 32 m, exposing changes in the strata of the limestone

Limestone

Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

 sides with depth. It is characterized by diverse and abundant fish

Fish

Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

 assemblages as well as a rich benthic

Benthic zone

The benthic zone is the ecological region at the lowest level of a body of water such as an ocean or a lake, including the sediment surface and some sub-surface layers. Organisms living in this zone are called benthos. They generally live in close relationship with the substrate bottom; many such...

 community of marine invertebrates, encrusting algae

Algae

Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

, sponges and soft corals.

External links

• Port Phillip Heads Marine National Park