Images captured by Europe's Mars Express spacecraft have revealed what could be a frozen sea, surviving as blocks of pack ice, just below the surface of the Red Planet, according to an international team of scientists. The apparent underground sea is located just five degrees north of the Martian equator, and as such, it would represent the first large body of water to be found beyond the planet's polar ice caps. According to a report in the New Scientist, scientists working with the high-resolution stereo camera instrument onboard Mars Express identified 'plates' that look similar to ice formations found near Earth's poles. One riddle posed by the frozen sea theory, should it prove correct, is how the ice plates managed to remain frozen given their proximity to the equator, where the sunlight should have melted them. The team suggests that a layer of volcanic ash a few centimetres thick may be protecting the ice. 'I think it's fairly plausible,' Michael Carr, a former member of the team from the US Geological Survey, told New Scientist. 'We know where the water came from. You can trace the valleys carved by water down to this areas. Maybe the ice is still there in the ground, protected by a volcanic cover, as they suggest,' he added. The team estimates the underground sea to be around 800 to 900 kilometres in size and an average of 45 metres deep. The depth estimate is based on a study of craters in the plates, which the scientists say appear too shallow for their diameters, suggesting that the ice is filling them up. By counting the number of craters in the ice, the scientists say that the plates appear to be around five million years old. The researchers have also offered a possible explanation for how the subterranean ice got there in the first place. Huge floes of ice, that were once floating in water, were covered by volcanic ash, thus preventing them from sublimating into Mars' thin atmosphere. The ice broke up and drifted before the remaining liquid water froze, but eventually all of the ice not protected by volcanic ash would have sublimated away, leaving behind just the ice plates. There are other unanswered questions surrounding the frozen sea theory, however. For example, if Mars has experienced relatively recent large scale sublimation, as the team suggests, why is there so little water vapour in the current Martian atmosphere. Ordinarily, it would be hoped that Mars Express' MARSIS radar - designed to look for subsurface water reservoirs and due to be deployed in May - could be used to confirm the presence of such a frozen sea. However, the instrument may experience difficulties in differentiating between ice and rocky soil. 'To preserve it, you've got to bury it. But if you bury it, you can't detect it,' concluded Dr Carr.