Strong evidence of liquid water below the surface of Mars has been announced. Radar studies of the subsurface have produced returns very like those from underground lakes in Antarctica. A separate project's radar was unable to find the same evidence, so the matter is unsettled, and in need of further confirmation.

A lake, some 20 kilometers in extent, though perhaps only a meter deep, situated about 2 kilometers beneath Mars' South polar region was indicated. This water would presumably be filled with minerals that lower its freezing point to below that of this strata of Mars. There were reported to be signs that there could be multiple lakes.

The presence of persistent lakes of liquid water would improve the chances for life on the Red Planet considerably. At this depth, it would be shielded from temperature extremes and radiation from space, which, due to Mars' very thin atmosphere, would be very serious problems for life at the surface.

Please find a link, below, to an article with further details:

https://www.cnn.com/2018/07/25/world/mars-subsurface-water-lake-evidence/index.html

If you go to my post on Science non SETI you can find a link to the Science article via Le Scienze online magazine.
Tullio

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It's slowly becoming obvious that there is a lot more water out there both on the moon and Mars as well as other places than was previously thought. That's good for the future of mankind off the earth.

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Bob DeWoody

My motto: Never do today what you can put off until tomorrow as it may not be required. This no longer applies in light of current events.

Here is a link to a CBC video, which includes an image of the subsurface radar returns that indicate water. The idea of 'basal melting' beneath Mars' South polar cap is discussed. Pressure from the ice would decrease the freezing temperature of water, and so keep t in a liquid state.

Other sources question whether a depth of only ~1.5 kilometers would be enough, on low-gravity Mars, to create sufficient pressure for this. The solution of various minerals in the water may act as 'anti-freeze', which can also lower its freezing point.

This high concentration of sodium-based minerals could present a problem for any life forms that might exist in the water. But given the almost fantastic ability of life to adapt to extreme conditions, already observed on Earth, it seems possible that life on Mars could exist in such underground lakes.

https://www.youtube.com/watch?v=d6hBHuKRw_c