Why basements stay cold even during summer? Why a room below the surface (such as basements) can stay cold all the time? How is it able to avoid the high increase of temperature and heat in hot days and periods?
 A: Dirt is an exceptionally good insulator. As seasons change, the temperature of dirt 5-10ft underground changes a lot less than the temperature of the air. The deeper you go, the less the change. So, in the summer a basement will be "cool" because the ground surrounding it is cool (relative to outside temperatures), and in the winter it will be "warm" because the ground surrounding it is warm (relative to outside temperatures), despite "cool" and "warm" both being the same temperature (probably about 50-60F if you're in the southern USA).

A: Basements can also be cool due to the moisture in the surrounding ground if near
a stream.  As the outside ground gets hotter near the surface, moisture is evaporated
making the ground cooler.  One of the advantages of having a basement with some
damp basement corners is that in the hot months the basement is dry and cool.
In ancient Persia ice houses, Yakhchāl where built above an underground aqueduct, qanat or underground stream to keep the ice house cool enough to store ice even when the
temperature outside might be 110 degrees Fahrenheit.
