If that water has to come from underground, yes, it would be cheaper and easier to just grab surface stone and crush it for material to make a heat battery. Al

If that water has to come from underground, yes, it would be cheaper and easier to just grab surface stone and crush it for material to make a heat battery. Also, water tends to make for a horrible heat battery as it is much more thermal-emissivity than rock particles. You need all sorts of additional insulation to retain the heat, whereas the sand will insulate itself.