Subsurface radar evidence is absent.
The MARSIS radar onboard Mars Express has found?....
...No evidence of liquid water at any depth it's signal is capable of reaching.
Equator or Martian poles?
It doesn't much matter what subsurface location on Mars is referred to since the thicker-than-expected crust would be more or less homogeneous.
Nick Hoffman's theory is getting more credence, while the Mars liquid-water/life optimists
find that evidence for their hypothesis is dying away.
Sorry, Robert Clarke.
BTW, we can all agree that Mars did, long ago, briefly possess pools or even small seas
of highly-acidic saltwater. But that's then, this is now.