Scientists divide underground rock formations into aquifers and aquifers. The aquifer and the aquifer are complementary, and we can think of the aquifer as a plate, and the aquifer on the plate is a sponge full of water. If the plate is gone, the water in the sponge will leak downward due to gravity, so the aquifer is very important for the preservation of groundwater.  

Groundwater is rarely polluted, so people in many areas use groundwater as domestic water, such as wells. So where does groundwater come from? Groundwater, like surface rivers, is mainly sourced from atmospheric precipitation in the sky---rain and snow. This water seeps into the ground from the surface and is stored in rock formations when conditions are right. When there is no refrigerator in the countryside, we may use the well to take water to soak watermelon or other fruits, I don't know if you have thought about it:

In the past, when tap water was not yet widespread, well water was an important source of water for urban and rural life. Some well water is clear and sweet, while others are bitter, cloudy and need to be clarified before drinking by using sediment filters for well water. But whether it is sweet water or bitter water, well water has a common feature - "warm in winter and cool in summer".

In winter, the water drawn out of the well is warm, not as cold as the open water; In the summer, the well water will become cold. Is there an air conditioner underground? Of course not, usually the well water temperature in winter will be a few degrees Celsius lower than in summer, but compared to the temperature difference between winter and summer on the ground, the change in well water temperature is very small, so it will give people the illusion of warm in winter and cool in summer.

The illumination of sunlight is the most important source of surface heat. But inside the Earth, sunlight doesn't have much of an impact on temperature. Only the surface of the earth's crust changes its temperature due to the influence of sunlight.

In summer, the sun is strong, but it cannot penetrate the soil. Because the well water is deep underground, only a small amount of heat can be transmitted, so the well water taken up in the summer is relatively lower than the water temperature of rivers and lakes on the surface, and it will feel cold when the hand reaches in.

Surface temperatures are low in winter. However, the formation below the ground does not directly emit heat, and the temperature will be higher than the ground. Therefore, the water from the well is warmer than the water on the ground and feels warmer.