I remember the root cellars on a few farms up in Ohio and Indiana. Here in Georgia, it's red clay over granite; not promising for good cellars.
Grandma's root cellar was a little white building next to the house, perhaps 10x20, but it was seen through a child's eyes and might be smaller. One or two very small windows, open the door to steps down with the above-ground section all shelves for canning jars. Down below was a frightening dark void, lit by a small bare bulb on a wire; mostly Grandma never bothered to turn it on. Man, those cellars frightened me!
But now I want one. Can't keep irish-type potatoes fresh very long here. We don't have a basement or even a crawl space to convert. It would have to be waaaaay over-built because I'm terribly allergic to mold, and therefore would be the cleanest one on the planet just for me to use. Is that even possible? There are these European ones that are all white-wash and tile.
If I had a nickel for every project I think up, I'd have enough money to pay somebody to do one for me!
I have spent a night in a cellar as a tornado ripped through overhead (first grade). I'm afraid this house wouldn't take a direct hit, so, there's another reason to build one.