New 3D mapping has revealed remarkable features of a sprawling submerged ridge east of Auckland - a feat one scientist says is the undersea equivalent of Captain Cook mapping the coastline of New Zealand.
Little was known about the Colville Ridge, stretching more than 200km towards Fiji, until a recent research voyage enabled marine geologists to map it in detail for the first time.
The insights, part of a project to create state-of-the-art maps of New Zealand's entire exclusive economic zone, could prompt a rethink about the origin of the vast ridge - and perhaps even that of its wider area in the South Pacific.
Three-dimensional imaging shows a mountainous ridge covered with jagged cones and with valleys that lie about 2km below the top of its peaks.
"If the ocean was drained from this area, you would see an impressive but eroded chain of mountains similar to the Tararuas in the southern part of the North Island," said the voyage's chief scientist, Dr Cornel de Ronde, of GNS Science.
"Essentially, what we have done is strip the ocean away and so people can see this part of the Earth's surface for the first time."
Until now, scientists had formed hypotheses about the ridge based on decades-old information, and most considered it a chain of extinct volcanoes.
"It's amazing to think this area is only 200km east of Auckland and ships have been sailing over it for more than a century pretty much oblivious to what lay beneath."
The new data revealed the ridge was far from entirely volcanic in origin, which had surprised some scientists.
"We set out to map what we thought was an ancient volcanic arc. We were surprised to find it wasn't dominated by volcanic rocks, but more a mixture of them with large tracts of sedimentary rocks."
The findings also backed the common belief that the ridge was once joined to the neighbouring Kermadec Ridge, but they were separated over millions of years by a huge rift.
Many large faults were found to be running through the area, mostly aligned northeast-southwest and dipping to the east.
It raised the possibility of seafloor minerals being present, as hot fluids containing dissolved metallic compounds tend to move along faults, especially where volcanic activity once occurred.
The possible presence of minerals will be investigated when Japanese and German research institutions, with New Zealand researchers, bring ships with autonomous underwater vehicles to study the area in more detail over the next two years.
For New Zealanders, the maps had revealed a new and meaningful part of our sovereign estate, Dr de Ronde said. There were perhaps four or five major ridge features in New Zealand's offshore territory and Colville was probably the least known of them.
"The Colville Ridge is a significant landmark on the face of the Earth - if you were standing on the plains either side of the ridge, the peaks would be 2km above your head."
He added: "It is the undersea equivalent of Captain Cook mapping the shoreline of New Zealand almost 250 years ago."