Planetary geologists led by John Murray of Britain's Open University said the evidence comes from pictures sent home by the European Space Agency's Mars Express orbiter.

High resolution, stereoscopic images reveal a flat, "plate-like terrain" in the region of southern Elysium Planitia, near the Martian equator, that appears remarkably like fields of pack ice on Earth, they say.

The "frozen lake" measures about 800 by 900 kilometers (500 by 560 miles) long and is probably about 45 metres (150 feet) deep on average, making it similar in size and depth to the North Sea.

"If our interpretation is confirmed, this is a place that might preserve evidence of primitive life, if it has ever developed on Mars," the group ventures in Nature, the British science weekly.

The apparent lake lies at the west end of Athabasca Vallis, an outflow channel leading from a trench called Cerberus Fossae.

Murray's team contends the water is the relic of an ocean created from sub-surface ice that was melted by volcanic activity and gushed to the surface along the Cerberus Fossae.

The water pooled and quickly froze, but almost immediately began to evaporate into Mars' low-pressure atmosphere.

The "lake" seen from the Mars Express pictures was possibly covered by a layer of volcanic ash and this acted like an insulative layer, slowing down the evaporative loss.

Over the past seven years, evidence has grown thanks to US and European space missions that Mars was once awash with water, one of the magical ingredients for creating life as we know it.

The seas eventually disappeared and residentual quantities of surface water remain locked up as ice at its poles.

But among the many unanswered questions, two big ones stand out: when were these oceans created? And could more water lie untapped under the surface?

A common view is that the big watery event probably happened several billion years ago, when Mars was an infant planet.

But Murray's belief is that the "lake" at southern Elysium is far more recent. It looks as if it were formed about three to seven million years ago, he contends.

Two other Mars Express analyses, also published in Nature, strengthen the theory of recent water flows on Mars.

These conclusions are based on glacial deposits formed in depresssions on the side of the volcano Hecates Tholus, and glacial activity on what is now tropical and mid-latitude regions on the Red Planet.

Evidence about water on Mars may be rapidly strengthening, but even so none has yet emerged that the planet harbours or has ever harboured life, even in the most rudimentary form.

The basis for Murray's speculation lies in the hope that beneath Mars' permanently cold crust, heat from volcanic activity may create a subterranean layer of warm water.

Over billions of years, this warm bath could help micro-organisms to evolve, even in negligible oxygen and in the absence of light, the authors argue.

Similar hardy creatures, called extremophiles, exist on Earth in deep-sea thermal vents and are common ancestors to many forms of life, they point out.

The research, which is published on Thursday, was first presented last month at a conference in the Netherlands on data sent back by Mars Express, where it created huge excitement.

If true, the frozen sea would be a prime candidate for a land mission to search for confirmation that life can exist elsewhere than on Earth.

All rights reserved. � 2005 Agence France-Presse. Sections of the information displayed on this page (dispatches, photographs, logos) are protected by intellectual property rights owned by Agence France-Presse. As a consequence, you may not copy, reproduce, modify, transmit, publish, display or in any way commercially exploit any of the content of this section without the prior written consent of Agence France-Presse.

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