Research indicates that ancient deposits of organic matter may have been converted to methane by microbes living in low-oxygen conditions.
The organic material dates back to a period 35m years ago when the Antarctic was much warmer than it is today and teeming with life.
Study co-author Prof Slawek Tulaczyk, from the University of California at Santa Barbara, said: "Some of the organic material produced by this life became trapped in sediments, which then were cut off from the rest of the world when the ice sheet grew. Our modelling shows that over millions of years, microbes may have turned this old organic carbon into methane."
Half the West Antarctic ice sheet and a quarter of the East Antarctic sheet lie on pre-glacial sedimentary basins containing around 21,000bn tonnes of carbon, said the scientists, writing in the journal Nature.
British co-author Prof Jemma Wadham, from the University of Bristol, said: "This is an immense amount of organic carbon, more than 10 times the size of carbon stocks in northern permafrost regions.
"Our laboratory experiments tell us that these sub-ice environments are also biologically active, meaning that this organic carbon is probably being metabolised into carbon dioxide and methane gas by microbes."
The amount of frozen and free methane gas beneath the ice sheets could amount to 4bn tonnes, the researchers estimate.
Disappearing ice could free enough of the gas to have an impact on future global climate change, they believe. More
