Weltevree, a Dutch company, is manufacturing and selling a prefabricated root cellar that can be dug into the ground and used almost immediately. They call it a GroundFridge, and it sells for between about $12,000 and $18,000 USD. I like the idea but not the price. They say that it holds about as much as 20 refrigerators.From their website: The Groundfridge is dug in and covered with the excavated soil from its new location. This covering layer of soil is about 1 meter thick and has sufficient insulating properties for the core temperature within the Groundfridge to barely vary. Furthermore, your Groundfridge is fitted with a ventilator. This battery-driven ventilator offers you the option of setting ventilation times to your desired specifications. Night venting will have a positive effect on reducing interior temperature, as letting the cold night air can see it drop rather rapidly. We recommend venting your Groundfridge at least one full hour every twenty-four hours.The temperature level inside your Groundfridge will more or less match the temperature on its outside at a depth of one metre below ground. Its interior temperature is affected by local factors such as soil type, ground water levels, exposure to sunlight, mound vegetation and the average outside temperature.
During peak temperatures the additional active cooler circulates and cools down the air inside the Groundfridge. The extra cooling power keeps the Groundfridge at a guaranteed and constant temperature which is adjustable to fit desired (professional) use.
Here is another option for making your own little root cellar: We offer a free plan for an earthbag root cellar.