A yellow submarine has helped solve a puzzle about one of Antarctica's fastest-melting glaciers, adding to concerns about how climate change may push up world sea levels, scientists said on Sunday.

The robot submarine, deployed under the ice shelf floating on the sea at the end of the Pine Island Glacier, found that the ice was no longer resting on a subsea ridge that had slowed the glacier's slide until the early 1970s.

Antarctica is key to predicting the rise in sea levels caused by global warming - it has enough ice to raise sea levels by 57 meters (187 feet) if it ever all melted. Even a tiny thaw at the fringes could swamp coasts from Bangladesh to Florida.

The finding from the 2009 mission "only adds to our concern that this region is indeed the 'weak underbelly' of the West Antarctic Ice Sheet," co-author of the study Stan Jacobs at the Lamont-Doherty Earth Observatory said in a statement.

West Antarctica's thaw accounts for 10 percent of a recently observed rise in sea levels, with melting of the Pine Island glacier quickening, especially in recent decades, according to the study led by the British Antarctic Survey (BAS) and published in the journal Nature Geoscience on the weekend.

Loss of contact with the subsea ridge meant that ice was flowing faster and also thawing more as sea water flowed into an ever bigger cavity that now extended 30 km beyond the ridge. The water was just above freezing at 1 degree C.

Satellite photographs in the early 1970s had shown a bump on the surface of the ice shelf, indicating the subsea ridge. That bump has vanished and the 7-meter (22-foot) submarine found the ridge was now up to 100 metres below the ice shelf.

Adrian Jenkins, lead author at BAS, said the study raised "new questions about whether the current loss of ice from Pine Island Glacier is caused by recent climate change or is a continuation of a longer-term process that began when the glacier disconnected from the ridge."

Pierre Dutrieux, also at BAS, said the ice may have started thinning because of some as yet-unknown mechanism linked to climate change, blamed mainly on mankind's use of fossil fuels.

"It could be a shift in the wind, due to a change in climate, that pushed more warm water under the shelf," he said.

The UN panel of climate scientists projected in 2007 that world sea levels could rise by between 18 and 59 centimeters (7 to 24 inches) by 2100, excluding risks of faster melting in Antarctica and Greenland. UN Secretary-General Ban Ki-moon has said the 21st century rise might be 2 meters in the worst case.