Images from the high resolution stereo camera on Mars Express showed of structures called plates that look similar to ice formations near Earth's poles.

These plates could indicate the first discovery of a large body of water beyond Mars' polar ice caps, the review said.

The team of researchers, led by John Murray of Britain's Open University, estimated the possible submerged ice sea at about 800 by 900 kilometresby 560 miles) in size and 45 metres (150 feet) deep on average.

The researchers said the evidence suggested that the plates, estimated to be about 5 million years old, were not just imprints left by ice that has now completely vanished.

While the site of the plates near Mars' equator means that sunlight should have melted any ice there, the team suggested that a layer of volcanic ash, perhaps a few centimetres thick, may protect the structures.

"I think it's fairly plausible," said Michael Carr, an expert on Martian water at the US Geological Survey in Menlo Park, California, who was not part of the team.

"Maybe the ice is still there in the ground, protected by a volcanic cover as they suggest," Carr said.

There is abundant evidence for the past presence of water on Mars but today the planet appears relatively dry, with ice confined to the planet's polar caps.

The discovery wwas to be presented Friday at the first Mars Express science conference in Noordwijk, the Netherlands.

In their paper the researchers traced a possible history for Mars' underground ice, saying it began with huge masses of ice floating in water that were later covered with volcanic ash, leaving the pack ice plates behind.

"If the reported hypothesis is true, then this would be a prime candidate landing site to search for possible extant life on Mars," said Brian Hynek, a research scientist at the Laboratory for Atmospheric and Space Physics at the University of Colorado at Boulder in the United States.

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