A "lost world" of undersea volcanoes has been discovered between Tasmania and New Zealand.
Some of the peaks, also known as seamounts, tower 3km above the deep sea ocean floor.
"The seamounts vary in size and shape, with some having sharp peaks while others have wide flat plateaus, dotted with small conical hills that would have been formed by ancient volcanic activity," said Dr Tara Martin of Australia's Commonwealth Scientific and Industrial Research Organisation (CSIRO).
Scientists on board CSIRO vessel the Investigator were studying phytoplankton - microscopic creatures vital for the ocean ecosystem, but threatened by climate change.
They detected a spike in phytoplankton activity and an increase in the number of birds and whales in the area, 400km east of Tasmania.
"We estimated that at least 28 individual humpback whales visited us on one day, followed by a pod of 60-80 long-finned pilot whales the next," said on-board scientist Eric Woehler. "We also saw large numbers of seabirds in the area including four species of albatross and four species of petrel."
Turning their instruments downwards, they found the undersea mountain chain.
"These seamounts may act as an important signpost on an underwater migratory highway for the humpback whales we saw moving from their winter breeding to summer feeding grounds," said Dr Woehler. "Lucky for us and our research, we parked right on top of this highway of marine life."
It's believed the mountains push undersea ocean currents closer to the surface, bringing nutrients with them - and that's why marine life is drawn to the area.
Scientists believe the seamounts are about 30 million years old, created when Australia, Tasmania and Antarctica split apart. The Investigator will return to the area in November and again in December with high-definition deep water cameras to take a closer look.
"We expect that these seamounts will be a biological hotspot year round, and the summer visit will give us another opportunity to uncover the mysteries of the marine life they support," said Dr Woehler.
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In 2015, the Investigator discovered four extinct undersea volcanoes off the coast of Sydney that dated back to when New Zealand and Australia split 50 million years ago.