The cooling of a space or substance below the environmental temperature. The art was known to the ancient Egyptians and people of India, who used evaporation to cool liquids in porous earthen jars exposed to dry night air; and to the early Chinese, Greeks, and Romans, who used natural ice or snow stored in underground pits for cooling wine and other delicacies. In the eighteenth and early nineteenth centuries natural ice cut from lakes and ponds in winter was stored underground for use in summer. The technique of mechanical refrigeration began with the invention of machines for making artificial ice. Great strides have been made in the twentieth century in the application of mechanical refrigeration to fields other than ice making, including the direct cooling and freezing of perishable foods and air conditioning for industry and human comfort.