re: Not "just" underneath the surface
The thing is, liquid water. Obviously there is no liquid water to be found on Mars today.
What everyone is looking for is signs of liquid water in the past along with life. Emphasis on past, PAST, as in everyone that has had a second degree education can determine that life cannot survive on present day Mars.
And yes, all of the liquid water/acid thereoff has been linked with volcanism/volcanic episodes.
Apologies that I do not have a degree as Mr. Hoffman, although I can assure you that I am quite literate. (Thank you.)