I want an old-fashioned root cellar. These are storage places for food (not just root vegetables) that do not require electricity for temperature or humidity control, but keep foods fresh throughout the year.
First, the temperature of a root cellar should be somewhere above freezing, but not more than 40 degrees (like a refrigerator). Temperatures depend on depth dug into the ground (the deeper you go, the less likely you are to dip below freezing), what direction it is facing (south-facing will get warmer) and whether you add insulation. Because our NY winters are cold, but not harsh, and the frost line gets to about 12-18 inches, it is a good idea to either dig a barrel root cellar so that food is stored below the frost line or have a protecting layer of mulch or insulation above the barrel to prevent freezing around it (so it would be mounded-up above ground).
Humidity is also important: too much humidity spoils food and too little dehydrates it. Different root vegetables require different humidity levels, but somewhere around 80-90 is usually safe. Maintaining the humidity level depends on the season (summers here are more humid) and the material of your cellar - soil is easy to control by either wetting it or drying it out with gravel layers; cement flooring or wood walls might need some water from time to time.
Ventilation is needed for the vegetables to breath and to help maintain temperature and humidity levels. Pipes in a barrel-dug cellar or windows in a hillside-dug cellar can assist the ventilation.
Finally, impenetrability from rodents and other pests is a must. You don't want little critters enjoying your harvest before you have a chance to.
We are using our deposito as our root cellar for now, but are trying to find an appropriate slope to dig into. There is also a room on the property already dug into a hillside, supposedly used by Jason's grandmother for refrigeration, underneath the High House study that stays cool all year but never freezes and has high humidity. If we can just rodent-proof it, we could be all set.