A cluster of submerged volcanoes has been discovered off Sydney, scientists said Monday, with the potential to help unlock the secrets of the sea floor between Australia and New Zealand.

The four extinct volcanoes were found about 250 kilometres (150 miles) offshore from Australia's biggest city last month during a search for larval lobster breeding grounds.

Australian National University volcano expert Richard Arculus said the volcanoes, which are spread across 20 kilometres, were "a window into the underlying mantle" beneath the seabed.

"They tell us part of the story of how New Zealand and Australia separated around 40 to 80 million years ago and they'll now help scientists target future exploration of the sea floor to unlock the secrets of the Earth's crust," he said.

Thought to be about 50 million years old, the volcanoes are at a depth of 4,900 metres (around 1,600 feet), leaving them undetected until now despite the largest being 1.5 kilometres across the crater rim and rising 700 metres from the sea floor.

However, a recently commissioned Australian research vessel, the Investigator, is equipped with sonar that can map the sea floor at that depth.

"I think every time we turn the spotlight on the sea floor we see things that we've never seen before," Arculus told AFP.

"My favourite phrase to people is 'We know the surface topography of Mars better than we know our backyard', and that's because there's no water in the way."

Arculus said that by examining the composition of the lava from the extinct volcanoes, scientists could be able to determine the composition of the underlying mantle layer.

"It's a bit like going through somebody's garbage bin and determining what they've been eating," he said.

The research conducted by the Investigator involved 28 scientists from several Australian universities as well as the University of British Columbia and the University of Auckland.

"The voyage was enormously successful," said its chief scientist Iain Suthers, from the University of New South Wales.

"Not only did we discover a cluster of volcanoes on Sydney's doorstep, we were amazed to find that an eddy off Sydney was a hotspot for lobster larvae at a time of the year when we were not expecting them."

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Australia and New Zealand