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A thriving ecosystem exists a mile and a half beneath Antarctic ice in a lake that has been cut off from the outside world for millions of years, scientists have discovered.

It is the home of micro-organisms that generate energy for growth from ammonium and methane.

Many belong to a primitive extended family of microbes distinct from bacteria, called the Archaea.

Lead scientist Professor John Priscu, from Montana State University in the US, said: "We were able to prove unequivocally to the world that Antarctica is not a dead continent."

The research, published in the journal Nature, is the first direct evidence that life can be found deep below the Antarctic ice sheet.

The team drilled through half a mile of ice to reach the sub-glacial Lake Whillans in January last year.

Archaea form one of the three great "domains" of life on Earth, the other two being bacteria and eukayotes - organisms with their DNA contained in a cell nucleus - to which humans belong.

Many of the sub-glacial archaea survived by harnessing energy in the chemical bonds of ammonium to drive metabolic processes, the research showed.

Another group of micro-organisms relied on the energy and methane locked into carbon.

The microbes originated in Lake Whillans and were not introduced by contaminated equipment, the scientists insisted.

"We went to great extremes to ensure that we did not contaminate one of the most pristine environments on our planet while at the same time ensuring that our samples were of the highest integrity," said Prof Priscu.