Sediments on the bed of Lake Ellsworth, which is several hundred meters (yards) below sea level and buried under 3 km (1.6 miles) of ice, may include bits of ancient seashells that could be dated to reveal when the ice sheet last broke up.

Experts say the West Antarctic ice sheet over the lake contains enough ice to raise world sea levels by 3-5 meters if it ever broke up - a threat to low-lying areas from Bangladesh to Florida, from Buenos Aires to Shanghai.

"Society needs to know the risk of a collapse," of the ice sheet, said Martin Siegert, of the University of Bristol and principal investigator for the mission that will also look for unknown life forms in a rivalry with Russian and U.S. scientists.

There are 360 known sub-glacial lakes in Antarctica - formed by heat from the Earth melting the bottom of the ice.

"One way to find out (the risks of collapse) is to know when it last happened," he said of the mission that has been in the planning stages for 16 years. "We are finally ready to hit the ‘go' button," he said.

"We set foot on the ice again in October and hope to bring samples to the surface in December 2012," Chris Hill, program manager at the British Antarctic Survey, said in a statement.

Siegert said no one knew the age of the West Antarctic ice. It might have broken up in naturally warmer periods about 125,000 years ago, 440,000 years ago or a million years ago - all times when sea levels were higher than today.

HIPPOPOTAMUS

Most worrying would be if the ice collapsed in the Eemian period 125,000 years ago when temperatures were slightly higher than now, hippopotamuses bathed in the Rhine and world sea levels were 4 to 6 meters higher than today.

That could be a sign that the ice sheet was very vulnerable to a collapse caused by man-made emissions of greenhouse gases.

Most scientists reckon that Greenland, with enough ice to raise world sea levels by 7 meters if it thawed over hundreds of years, is more vulnerable than West Antarctica.

The far bigger East Antarctic ice sheet, with enough ice to raise sea levels by more than 50 meters, is coldest and most stable. Sea levels rose by 17 cm (7 inches) last century but many scientists say the rate might pick up to a meter this century.

Siegert said the plan to drill into the lake sediments had received little attention compared to the goal of seeing if microbial life had evolved in the darkness under the ice.

That might increase the chances of finding life elsewhere in the solar system, such as on Jupiter's ice-swathed moon Europa. The scientists aim to use a hot water drill and take samples in a sterilized titanium container.

Russia drilled through to the Vostok Lake in East Antarctica early this year and its scientists plan to return in 2013 to get samples.

U.S. scientists plan to sample the Whillans sub-glacial lake "around mid-January" 2013, said John Priscu of Montana State University.

He said that the mission would send down a robot vehicle to sample the lake, keeping the borehole open for about two weeks unlike the British plan for faster sampling over a few days.

"The Russians have no clean samples from Vostok this time," he told Reuters. "The only samples they have that I am aware of is water, mixed with kerosene drilling fluid."

Siegert played down the three-nation competition. "We don't really regard it as a race. But all of science is driven by people who want to be first," he said.

(Reporting By Alister Doyle; Editing by Robin Pomeroy)

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