A rare species of stingray, a 2,000-year-old coral and extinct underwater volcanoes are among hundreds of discoveries found under the ocean by a scientific expedition off the coast of Australia.

Marine experts say that the discoveries shine a new light on what they say are some of the most biologically important oceanic habitats in the world.

In the icy waters of a Southern Ocean marine park, 100 nautical miles off the coast of Tasmania, scientists found more than 270 new marine species — including fish, crustaceans, molluscs, sponges and corals — and 80 previously unknown seamounts or underwater mountains.

The undersea mountains, which measured half a mile high and 15 miles (25km) wide, form “rainforests of the deep”. For the first time scientists also laid eyes upon underwater canyons larger than the Grand Canyon in Arizona.

The £600,000 scientific expedition was led by Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO), the national science agency.

Announcing the discoveries in the Tasmanian capital, Hobart, the CSIRO scientist Kate Wilson said that the oceans remain a mystery — more is known about the surface of Mars.

“In Australian waters, for example, more than 40 per cent of the creatures brought to the surface by our scientists on a voyage of discovery have never been seen before,” she said.

Scientists said that only a tiny proportion of Australia’s oceans had been explored in such a way and that they could only speculate on the biodiversity hidden under the water.

The discoveries were made during two voyages in November 2006 and April 2007. The expedition focused on reserves in the Southeast Commonwealth Marine Reserve Network, which covers 226,000 sq km (87,300 sq miles) of the Southern Ocean.

Scientists used special sonar equipment and video technology on board the vessels to create a map of the seabed and detected the 80 previously unknown underwater mountains and 145 new undersea canyons.

Nic Bax, the CSIRO marine resource management expert,said that some of the oldest discoveries included 2,000-year-old living black coral, bamboo coral more than 300 years old, and fish about 200 years old. Carbon dating was used to determine the age of the deep-sea treasures. In total 123 underwater mountains were also revealed.

Of the discoveries about 70 per cent had never been seen by humans before. “We have no idea how many species there are, and most of the species we get we only catch once,” Dr Bax said. “So who knows how many more are down there,” he added.

In the cold depths of the Southern Ocean “things grow quite slowly so when you’re looking at a coral which is maybe two metres high, it may also be 300 years old or more,” said Dr Bax. “These findings indicate how little we know about marine species around Australia and the deep sea.”

A live stingray, from a species of which only one dead specimen had ever been found in Australian waters, was also captured, and a glass sponge, so delicate that it could not be brought to the surface without breaking it.

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