Build a root cellar with an unused septic tank by setting the tank in an existing bank of soil, cutting a door in the concrete, backfilling the area with more soil and installing a heavy, wooden door. Add vents to allow the cellar to fill with cool air.
Root cellars use the insulation of the soil and low outside temperatures to lower the temperature of the structure to between 32 and 40 degrees Fahrenheit. Installing vents in the roof using the openings on the side of the tank and cutting a hole in the lid to install the floor level vent allows heavy, cold air to settle to the bottom of the tank.
Check with local septic tank manufacturers for tanks that do not meet their quality standards but that are still usable as a root cellar. Chips and cracks that allow liquids to seep out of the tank work well for this purpose. Rent a gasoline-powered concrete saw to cut the door opening and a rotary hammer drill for cutting an opening in the floor for drainage and roof for a vent.
Root cellars provide the storage for root vegetables at a temperature and humidity level that slows spoilage. They use natural cooling and insulation to maintain these levels.