According to recent findings, the water that once existed on Mars’ surface could be found underground. Credit: Kevin Gill
Recall... Recall... Recall...
Blue Sky on Mars.
The particles out of which Mars was formed had appreciable amounts of water. You might expect a fair amount of water locked up in minerals or at some depth. Of course, there are far less minerals on Mars than on earth as robust life and oxygen contribute to lots of minerals. The rocks from the pictures do indicate plenty of surface water at one time and deep water accompanies that. Volatiles like water are largely gone from the surface. However you might find a biosphere with abundant water at depth. JMHO
Related item: Astronomy picture of the day of the subsurface water of Mars.
