We now know that the planet Mars once had an enormous ocean that covered over a third of its surface, about 3.5 to 3.7 billion years ago, and now we also may be able to examine some remnants of it. The Mars Exploration Rover "Spirit", which is currently stuck and unable to move on the Martian surface, has nonetheless been able to glean much valuable data about its current location. It's found evidence of subsurface water beneath it, which may be snow melt or permafrost, but could possibly also be traces of the great body of water which once ruled the red planet.
