A huge, frozen sea lies just below the surface of Mars, a team of European scientists has announced.
Their assessment is based on pictures of the planet's near-equatorial Elysium region that show plated and rutted features across an area 800 by 900km.
The team think a catastrophic event flooded the landscape five million years ago and then froze out.
They tell a forthcoming edition of Nature magazine that sediments covered the ice, locking it in place.
Large reserves of water-ice are known to be held at the poles on Mars but if this discovery is confirmed by follow-up observations, it would be a first for a region at such a low latitude.
"It's been predicted for a long time that you should find water close to the surface of Mars near the equator," Jan-Peter Muller, from University College London, UK, said.
"This is an area where there are a lot of river features but no-one has ever seen a sea before, and certainly no-one has ever seen pack ice before," he told the BBC News website.
The interpretation is based on images taken by the High Resolution Stereo Camera on Europe's Mars Express spacecraft. These show extensive fields of large, platy features - reminiscent of the fractured ice floes found in polar regions on Earth.
Finding exposed ice at the equator would be unlikely. Very low pressures on the planet would lead to sublimation - the ice would erode over time straight to water vapour.
But the research group, led by John Murray, from the Open University, UK, tells Nature that a crust of dust and volcanic ash, perhaps just a few centimetres thick, has prevented this happening.
"The story runs that water flowed in some kind of massive catastrophic event; pack ice formed on top of that water and broke up, and then the whole thing froze rigid," explains Professor Muller.
"Large amounts of dust then fell over that area. The dust fell through the water and on top of the pack ice, which explains why the pack ice is a different hue to the area around it."
The water that formed the sea in the southern Elysium, five degree north of the equator, appears to have originated beneath the surface of Mars, erupting from a series of fractures known as the Cerberus Fossae.
Many of the features seen by Mars Express have also been pictured by the Mars Orbiter Camera on the US Mars Global Surveyor probe.
Further data is now required to support the initial observations but already other scientists think the interpretation is reasonable.
Mars Express is due to deploy its Marsis experiment in May
"I think it's fairly plausible," commented Michael Carr, an expert on Martian water at the US Geological Survey in Menlo Park, California, who was not part of the team.
He told New Scientist magazine that a past water source north of the Elysium plates had previously been suspected.
"We know where the water came from... You can trace the valleys carved by water down to this area."
Mars Express has now been in orbit around the Red Planet for a year.
It has already confirmed US observations that substantial water-ice lies at the poles, on its own and mixed with carbon dioxide ice and dirt.
The probe will soon deploy its Marsis (Mars Advance Radar for Subsurface and Ionospheric Sounding) instrument, which has been designed to find the planet's subterranean permafrost.
This underground ice is thought to be the major reservoir for water on Mars today.
However, the way the instrument is set up means it may not be able to see the Elysium sea because it is simply too near the surface. Only if the ice mass extends down many tens of metres will it be able to detect the sea-bottom boundary.
The presence of so much recent (in the geological timeframe) liquid water will excite the speculation that life could have thrived in this area.
"The fact that there have been warm and wet places beneath the surface of Mars since before life began on Earth, and that some are probably still there, means that there is a possibility that primitive micro-organisms survive on Mars today," Professor Murray said.
"This mission has changed many of my long-held opinions about Mars - we now have to go there and check it out."
Professor Muller added: "What we'd like is for the European Space Agency (Esa), with UK support, to send its next lander there."
Details of the frozen sea were given at the Mars Express science conference, taking place at Esa's European Space Research and Technology Centre (Estec), in Noordwijk, the Netherlands.