A science paper released today and available for download [pdf] cites evidence from about two dozen deep impact craters located from the equator to 37 degrees north latitude that Mars has a ground ice table at an elevation that also corresponds to other shoreline features. From the abstract:
Observations in the northern hemisphere show evidence of a planet‐wide groundwater system on Mars. The elevations of these water‐related morphologies in all studied basins lie within the same narrow range of depths below Mars datum and notably coincide with the elevation of some ocean shorelines proposed by previous authors.
The image above and on the right shows the middle stage of their conceptual model for the evolution of these deep basins and how that evolution results in many of the geological features seen in many places on Mars, such as the features I have highlighted on Behind the Black previously here and here.
From their conclusion:
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