I've heard somewhere that during summer the temperature of ground water is lower than that on the Earth while during winter the temperature of ground water is higher. How is this possible ?
Soil is a bad conductor of heat. So, the layers of the top surface of earth act as insulators.
During summer, they insulate the underground water and soil from the heat radiation from the sun, and from the general hotter atmosphere, hence underground water is colder.
In winter, they again act as insulating layer and prevent the heat from underground to escape into the cold outside air. Thus, underground water can be warmer than outside.
One noteworthy point is that the underground water temperature remains fairly constant throughout the year, (for a given geographical region), because the water table is quite below the ground, and hence it is insulated from the outside atmosphere.
It feels colder during summer, because outside water is hotter and it feels warmer during winter because outside water is colder .