Before the development of artificial refrigeration, humans already knew how to maintain low temperatures in simple devices using natural ice and snow, that is, utilizing natural cold sources.
In China, the use of natural ice for food preservation dates back approximately 3000 years. The *Book of Songs*, dating back to the 7th century BC, contains verses about collecting, storing, and using natural ice to preserve food.
Even in modern times, people still utilize natural cold sources such as ice, snow, and groundwater. Refrigeration devices cooled by natural or artificial ice can only achieve limited low temperatures, and their technical and hygienic conditions are poor, making it difficult to meet diverse requirements. Modern refrigeration devices all utilize refrigeration machines for cooling.
