Root cellars are a very old-fashioned way of preserving food, and it can be one of the simplest if you have the right location for it. All a root cellar is is a cold, dark location that also has the right level of humidity to allow some types of fruit and vegetables to last months without any processing at all.
Pros and Cons to Using a Root Cellar
The biggest positive aspect of using a root cellar is the simplicity. Once you have the right location and conditions set up, you just store your produce in baskets or buckets and leave as-is in the cellar until you are ready to use them.
The problems come from trying to get the right conditions for what you are going to store and that this method will only extend the life of your food for a few months or possibly to a full year if you are lucky. That's fine for standard food preservation from one season to the next but it usually isn't the right type of time frame that you need for a good emergency food storage system.
There are also limits one what kind of food will store this way. Most root vegetables (potatoes, turnips, beets) and apples are well-known for great root cellar storage.
The specifics of where or how you build a root cellar are less important that you are able to achieve the right environment once it's in use. Not all vegetables will store best under the same conditions but you should have a space that stays between 32 and 38F (0 and 3C) without dipping below freezing. Humidity should also be very high, around 90%. The temperature is much easier to maintain naturally during the winter months, which is why this method works so well as a yearly method to save fresh produce until the next season. Once you are storing through the summer, most underground places will be too warm.
You also need to allow for ventilation. Gases can build up that will hasten the ripening (and degradation) of your food so you have to have some sort of venting to the outside for fresh air. Because air should be moving, you will have to have a source of moisture present to keep the humidity level up. A room that has a dirt floor will usually be damp enough but if you do not have that source for moisture, then you can keep a few buckets of wet sand among your stores. Just add more water to it as the sand dries out. Keeping a hygrometer in the room to measure humidity is a very good idea.
Take note that other types of storage food (canned goods for example) should not be stored along with your produce in a root cellar. The high moisture levels will rust cans very quickly so reserve your root cellar for unpackaged foods only.