Pockmarks are crater-like structures on the seafloor created by fluids and gases as they erupt into the water through ocean-bottom sediments.
Researchers from New Zealand, Germany and the United States said three newly discovered pockmarks -- the largest of which is 7 miles long, 4 miles wide and more than 300 feet deep -- may be twice the size of the largest pockmarks previously recorded in scientific literature, New Zealand news website Voxy.co.nz reported.
Scientists said they believe the structures, at a depth of about 3,200 feet under the ocean surface, are the ancient remnants of vigorous degassing from under the seafloor into the ocean, although there is currently no sign of gas being emitted from them.
Gas release from the larger pockmarks may have been sudden and possibly even violent, with a massive volume being expelled into the ocean and atmosphere within hours or days, the scientists said.
"It's most unusual for scientists to encounter seafloor structures of this size and complexity," New Zealand marine geophysicist Bryan Davy said. "They are big enough to enclose the Wellington city urban area, or lower Manhattan."
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