. . . or to put it another way . . .
A liquid will spread to take the shape of any container which it's poured into.
While the salt in the cellar may spread to roughly take up the shape of the cellar, consider what would happen if you emptied that cellar full of salt into the middle of a saucepan. You'd just get a pile of salt in the middle of the pan. (i.e. the salt wouldn't spread to take the shape of the saucepan, as would have happened if you'd have had water in the cellar, rather than salt).
Chris