By using the High Resolution Stereo Camera on Mars Express, scientists think they've found a frozen sea just five degrees north of the Martian equator. It's thought the ice is held in gigantic "plates", covering an area some 800 kilometers square and 45 meters deep. The hypothesis is the plates formed when huge masses of ice floating in liquid water got covered in volcanic ash. The rest of the ocean froze up around them, and then evaporated away, leaving the plates behind.

The evidence is, of course, contested, with some scientists pointing out similar formations in other areas on Mars are clearly formed of lava and not ice.