"It is really, really big and it looks different from the others we have studied," said Dr Paul Tyler, of the UK's Southampton Oceanography Centre.
Vast towers of mineral deposits have been discovered in the middle of the Atlantic.
Researchers diving in the mini-sub Alvin were so astonished by the scale and beauty of the field they have dubbed it the Atlantic's "Lost City".
"We thought that we had seen the entire spectrum of hydrothermal activity on the seafloor, but this major discovery reminds us that the ocean still has much to reveal," said Margaret Leinen, of the United States National Science Foundation.
National park
"These structures, which tower 55 metres (180 feet) above the seafloor, are the largest hydrothermal chimneys of their kind ever observed," said Deborah Kelley, a University of Washington geologist who was part of the expedition.
"It is really, really big and it looks different from the others we have studied," said Dr Paul Tyler, of the UK's Southampton Oceanography Centre.
But it was Duke University structural geologist Jeff Karson who probably caught best the mood of excitement created by the discovery when he said: "If this vent field was on land, it would be a national park."
The researchers say that the most surprising aspect of the new find is that the venting structures are composed of carbonate minerals and silica, unlike most other mid-ocean-ridge hot-spring deposits which are formed by iron and sulphur-based minerals.
Life support
Also, other vents in the Atlantic have a rich population of shrimps and bacteria but they appear to be absent in this field.
The Lost City field was discovered unexpectedly while researchers were studying the geological and hydrothermal processes that built an unusually tall, 3,700m (12,000ft), underwater mountain. The region under investigation is 1,600 kilometres (1,000 miles) south of the Azores.
In the area, rocks called serpentinised peridotites, and rocks crystallised in under-seafloor magma chambers, have been uplifted from beneath the seafloor along large faults.
Donna Blackman, a geophysicist from the Scripps Institution of Oceanography and chief scientist of the expedition, said: "The venting towers are very spectacular and, although they bring up a whole new set of questions, we will learn about the evolution of the mountain itself as we study the vents carefully in the future."
Scientists have seen hundreds of overlapping flanges on the chimneys that some say are reminiscent of hot spring deposits in Yellowstone National Park.
"By studying such environments, we may learn about ancient hydrothermal systems and the life that they support," said Dr Kelley.
The research vessel Atlantis, the mother ship for the deep-diving submersible Alvin, was used as the base for the underwater expedition.