Comment by Aurornis

> I've always wondered why we don't build homes with a buried tank of water used as heat storage

Skip the tank completely. Use the ground directly. This is what geothermal heating does.

Drill a deep hole and drop tubes into it. Use a heat pump to pump heat into or out of the ground. There is so much easily accessible thermal mass in a borehole that you don’t need to deal with a giant underground water tank