"We usually think of ice sheets like cakes -- one layer at a time added from the top," said Robin Bell, a geophysicist at Columbia University's Lamont-Doherty Earth Observatory, who co-authored the study in the journal Science.

"This is like someone injected a layer of frosting at the bottom -- a really thick layer."

The team flew over the vast East Antarctic ice sheet, setting out to unravel the mystery of the so-called "ghost mountains" hidden underneath. Known formally as the Gamburtsev Mountains, the range buried in the ice pack near the South Pole spans an area larger than the European Alps.

Using ice-penetrating radar, laser ranging systems, gravity meters and magnetometers, they were able to map three-dimensional images of Dome A, a 13,800-foot-high (4,200-meter) plateau which crowns the East Antarctic ice sheet.

They also found unusual activity below the ice sheet, with formations that looked like beehives as a result of ice melting and refreezing, an activity that pushes and warps the ice sheet upward.

Researchers were surprised to see that the images produced by their geophysical tools "show that the refreezing deforms the ice sheet upward," the study said.

"We did not think that water moving through ancient river valleys beneath more than one mile (1.6 kilometers) of ice would change the basic structure of the ice sheet," said Bell.

A full 24 percent of the ice sheet around Dome A is made up of refrozen ice, the study said.

In some places, where ice melted and refroze at a quicker rate than it could be formed from above, more than half of the ice's thickness appears to have come from below.

Scientists think the deep buried ice may melt because ice layers from above are insulating it, making the under layers more prone to heat from friction or warmth radiating from rocks below the ice.

Understanding how the ice sheet acts is crucial to the study of climate change. Scientists believe this act of freezing, melting and refreezing has been going on since ice first encased the Antarctic 32 million years ago.

The East Antarctic ice sheet spans 3.8 million square miles (9.8 million square kilometers), about the same area as the United States.

"The Antarctic ice sheet holds enough fresh water to raise ocean levels 200 feet (61 meters); if even a small part of it were to melt into the ocean, it could put major coastal cities under water," the study said.

The ongoing research is part of an effort that found lakes beneath the ice sheet in 2006, and includes scientists from the United States, Britain, Germany, Australia and China.

Scientists from the Chinese team hope to be able to drill into Dome A in the coming two or three years to find traces of long-ago climate shifts, and possibly ice that is over a million years old.

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