re: Ground "water"
Enceladus has an active (maybe more, we don't know) cryovolcano that spews ice particles. As to what's going on below it, we don't know yet.
Same can be said for Mars, although there seems to be a lot more tidal heating on Enceladus than what's going on below Mars at this point (mh readings and observation).
As some have said, caves are not uncommon occurences surrounding volcanoes.
I just hope that the creative minds at JPL can come up with another mission to study this area (a rover with a cable and bucket, et al).
Kiddying aside, there's always hope.