The finding challenges the widely held assumption that sub-glacial lakes evolved in isolated conditions for several million years, and it raises the possibility that water from deep within the ice's interior may have generated huge floods that reached the ocean in the past - and may do so again.

"Previously, it was thought that water moves underneath the ice by very slow seepage," said team leader Duncan Wingham, of the University College London. "But this new data shows that, every so often, the lakes beneath the ice pop off like champagne corks, releasing floods that travel very long distances."

The researchers found anomalies in the ice sheet's surface elevations using ultra-precise measurements from ERS-2's radar altimetry and radar interferometry. Close inspection of one anomaly revealed an abrupt fall in ice-surface elevation with a corresponding abrupt rise some 290 kilometers (180 miles) away.

The only possible explanation for these changes is a large flow of water was transferred beneath the ice from one sub-glacial lake into several others, the researchers said.

Radar altimeters - originally designed to measure sea-surface height by beaming down 1,800 radar pulses per second, then recording how long their echoes take to bounce back - have been highly successful in measuring large-scale ice surface changes over time.

Radar interferometry, known as InSAR, is used to measure the spatial pattern of the changes precisely. Together, they form a powerful instrument to investigate the physical causes of changes in the ice surface.

Wingham, who is director of the university's Center for Polar Observation and Modeling, said the sensors proved essential for the study.

"With radar altimetry we can tell how much the lake levels fell and rose, and that it occurred between late 1996 and early 1998, but we cannot determine the area, which is vital for investigating the physics of the flood because we need to know the volume of water involved," he said.

For that reason, he added, " we turned to ERS-2 InSAR interferometry, which has excellent spatial resolution. With it, we were able to image the area of at least part of one lake, which was enough to give us an idea of the volume, and with this we could get at the rate of the flow."

Sub-glacial lakes in Antarctica first were identified in the 1960s. Since then, more than 150 have been discovered, but thousands more may exist, because much of the bed of Antarctica remains unexamined. The team focused its study on the Dome Concordia region in eastern part of the continent, which features more than 40 known lakes.

Several groups, under the umbrella of the Scientific Committee on Antarctic Research, currently are investigating the prospect of drilling down to sub-glacial Antarctic lakes where ancient life is thought to exist. Because of this discovery, however, scientists might have to review their plans.

"A major concern has been that by drilling down to the lakes new microbes would be introduced," Wingham said. "Our data show that any contamination will not be limited to one lake, but will over time extend down the length of the network of rivers. We had thought of these lakes as isolated biological laboratories. Now we are going to have to think again."

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