A crater in the equatorial region of Mars has been found to contain exposed surface ice. That ice appears to be part of an extensive ice sheet which extends well beyond the polar regions of Mars.  The crater was caused by a recent meteor, and the exposed ice was eventually covered again by Martian surface dust.

So, it's now established that vast sections of Mars are ice sheets covered in regolith dust. The ice below the surface is exposed by meteorite impacts, then quickly covered again by the prevailing dust storms on the red planet.  I wonder whether the extent of the ice below the surface might be even greater still. Perhaps Mars is more like one of the Gallilean moons of Jupiter. Perhaps not as obviously ocean-friendly as Europa, but more like Callisto and Ganymede?  In those cases frozen sub-surface ice is gently warmed by the proximity to Jupiter (the same scenario for a warmed habitable world orbiting a Dark Star).  Mars does not have such a massive companion to warm the sub-surface ice into an ocean.  But...it is large enough to have volcanic activity, as the considerable calderas on Mars indicate. 
 
Surface features on Mars tend to bat down the idea of active recent vulcanism, because there are large swathes of ancient craters which should have been filled in long ago under that scenario.  But it certainly seems to be reasonable to paint a picture of subsurface ice sheets covering Martian oceans warmed by underground geothermal activity.  Meteorite impacts crunching through the surface ice and releasing underground water might explain some of the Martian anomalies of dried riverbeds.  If the meteorite that uncovered the ice in this case had been bigger, we might have witnessed just such an effect!
 

References:

2)  Andrea Thompson, "Water Ice Exposed in Mars Craters" 24th Sept. 2009, http://www.space.com/scienceastronomy/090924-mars-crater-ice.html with thanks to David