Not "just" underneath the surface
1) The ESA radar didn't find evidence of liquid water in its ~5 km depth of investigation.
2) No IR hot spots or active volcanoes have ever been found on Mars.
3) All the geological evidence is that Mars is now cold and dry and dead to depths of 8-20 km
4) All the major fluid outbursts of the past have been linked with major volcanic episodes.
Integrating the above, it is very likely that there are pools of mineral-laden (read "Acidic") water under the roots of the old volcanoes, but it isn't going anywhere in a hurry. You'll all have to wait for Mars' next volcanic episode, which could be 100 or 500 million years in the future.
Or we could go there and drill for evidence, but you'll need a BIG drilling rig to get that deep...