How ancient people made ice without electricity

Modern day tasty pappingsu in Korea

So as I was sitting at home watching K drama Sungkyunkwan Scandal, there was a scene where a teacher was handing out "pappingsu", a delicious treat made of shaved ice, red bean and rice cakes. Several centuries ago before modern technology, ice was valuable and very expensive. It got me wondering, how did people back then, especially those in countries with warm climate produce and preserve ice???

The Romans' ice-making method required that you be in the desert, or at the very least in an area with low humidity to facilitate heat loss and lower temperatures at night. The method described below was used a lot in North Africa and Palestine, for example.

The Romans would put water into a pit that was well-insulated with straw. The pit would be covered with highly polished shields during the day, to reflect the heat of the sun, while at night the pit would be uncovered so that the water within could lose the maximum thermal energy. Ice often began forming in the evening, and would typically be ready for harvesting by 3 or 4 a.m. Once harvested, the ice would be taken to the nearest icehouse for storage.

The more water placed in the pit, the more resistant it is to freezing overnight. But this method does work, without refrigeration, and without electricity, and was used by the Romans to augment their seasonal ice harvests.

Source: Making Ice in Ancient Rome, Everything2.com

My thought: It still baffled me how people back then figured out the process of thermal energy. I had to go back and re-watch the documentary "Ancient Astronauts". With all the science and work put into it just to cultivate the impossible, I can't imagine how expensive ice would be back then and only the filthiest of the richest could afford it.