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Robot vehicles have mapped a large underwater volcano off the north-eastern coast of New Zealand, revealing new mineral-rich underwater structures which could one day be mined.

New Zealand scientists yesterday returned to Auckland on the Sonne, a German research ship, after a three-week voyage and international collaboration to explore the Brothers volcano, 400km northeast of White Island and about three times its size.

Project leader Cornel de Ronde, of GNS Science, said it was the first time a submarine volcano had been mapped and surveyed for hydrothermal venting in such detail.

"It's like comparing a view of Mt Ruapehu from the air to driving around it in a four-wheel-drive and looking out the window."

Dr de Ronde said the high resolution topographical information was achieved through the use of an Autonomous Benthic Explorer (ABE), supplied by the Woods Hole Oceanographic Institution in the United States.

The 5m-long underwater machine resembled a spacecraft, he said.

"It looks like the Starship Enterprise ... it operates like a missile flying over the volcano."

The pre-programmed craft had travelled 161km inside the crater of Brothers volcano for 96 hours, following a grid pattern, mapping and collecting a wide range of information about the seafloor.

Dr de Ronde said the ABE sensors had produced the most comprehensive view of hydrothermal venting ever observed on a submarine volcano, of which the deepest part of the crater was 1850m below the sea surface.

As a result the expedition had produced stunning maps of the seafloor and a three-dimensional graphic of the 4km by 3km crater, he said.

Dr de Ronde said two vents, like hot springs on the sea floor, were already known of, but the ABE had found another where mineral-rich hydrothermal fluids were released from volcanic chimneys.

When the hot fluids, about 300C, mixed with the cool ocean, about 3C, the metals precipitated and formed black "smoke" plumes.

"It looks like a stack in an industrial estate."

Dr de Ronde said the chimney stacks contained metals such as copper, zinc, lead and gold.

Although it had never been done, it was possible they could be mined. Exploration companies were already investigating the possibilities, he said.

The $1.3 million expedition, split between New Zealand, the United States and Germany, had produced a wide range of other information about the seafloor.

"Robotic vehicles allow us to increase our scientific output dramatically."

The ABE resolution was at least 10 times greater than that achieved with previous mapping techniques using multibeams from the sea surface.

Dr de Ronde believed the future of deepsea exploration would revolve around robotic vehicles.

"Considering New Zealand has one of the largest exclusive economic zones in the world, perhaps it is time the country took a lead in this exciting new frontier."

New Zealand researchers have been exploring since 1999 the chain of volcanoes that formed the Kermadec Arc, spanning 2500km between New Zealand and Samoa.