The recent discovery of possible ancient shorelines in the martian northern plains has suggested to some scientists that the release of water associated with the formation of the outflow channels may have led to the temporary formation of large ice-covered lakes or seas. Some have even suggested that if the channels were active simultaneously, the resulting discharge may have been sufficient to have formed an ocean in the low-lying northern plains, covering as much as 30% of the planet's surface to a depth as great as several kilometers. This hypothesis is illustrated in this painting by Michael Carroll. While most investigators now acknowledge the likelihood that smaller bodies of water once may have resided on the martian surface, the possible existence of a “Northern Ocean” remains an issue of active debate.
Painting by Michael Carroll, reprinted with permission of artist