An international group of scientists has discovered three ancient sea-floor craters off the coast of New Zealand.
New Zealand, German and American scientists believe they have found the world's biggest pockmarks on the Chatham Rise, about 500 kilometres east of Christchurch.
Possibly formed by volcanic activity, pockmarks are created by fluids and gases erupting through sediments into the ocean.
The largest one found on the Chatham Rise is 11 kilometres across at its widest - enough to enclose Wellington city - and 100 metres deep.
A GNS Science marine geophysicist, Bryan Davy, says they it's rare for them to be so large.
He says the pockmarks could have been created by an enormous release of gas from the seafloor.
He says there is currently no sign of them emitting gas.