Giant craters found off the South Island coast

Gas release from the larger pockmarks may have been sudden and possibly even violent. Image / GNS Science

Giant craters found on the sea floor off the South Island coast may be the world's biggest pockmarks, say scientists.

Three giant pockmarks - the largest 11km long and 6km wide and 100m deep - were discovered at Chatham Rise, 500km east of Christchurch, by New Zealand, German and American scientists.

The structures, caused by fluids and gases erupting through sediment into the ocean, were possibly twice the size of the largest pockmarks recorded in scientific literature, GNS Science said.

"It's most unusual for scientists to encounter sea floor structures of this size and complexity. They are big enough to enclose the Wellington city urban area, or lower Manhattan," said GNS Science marine geophysicist Bryan Davy.

The three structures are part of a field of many thousands of smaller pockmarks that extends east from Banks Peninsula for several hundred kilometres.

Gas release from the larger pockmarks may have been sudden and possibly even violent, and volcanic activity was one possible cause of the release of gas, said the scientists.

There was no sign of active gas systems in the larger pockmarks but smaller ones in shallower water appeared to have been sporadically active.