The Mars ocean hypothesis proposed that the Northern Plains of Mars, approximately one third of Mars surface, was once covered by an ocean of liquid water. Given that the Northern Plains of Mars are the youngest portion of Mars surface- as evidenced by their sparse cratering relative to most of the planetÂs surface- the Mars paleo-ocean appears to have endured for most of Mars geologic history in either liquid or solid form. The paleo-ocean, named the ÂMalacandrian OceanÂ or ÂOceanus Borealis,Â would have filled the Vastitas Borealis basin in the northern hemisphere of Mars, an area which lies 4Â5 km (2.5Â3 miles) below the mean planetary elevation. This would have begunat a time period of approximately 4.0 billion years ago. Evidence for this ocean includes geographic changes in the terrain smoothness at a common elevation, geomorphology that resembles ancient shorelines, and the chemical properties of the Martian soil and atmosphere in the Northern Plains region. Mars would have required a denser atmosphere and warmer climate to allow a liquid ocean to remain at the surface.