Why is a cool basement often damp in the summer?

In a basement, the earth itself (and sometimes bedrock or large boulders) prevent extreme temperature extremes from occurring. In the summer, the basement would then be relatively cool compared to the outside temperature. Also in summer where people have basements, it gets humid as well as warm. When this humidity gets into the basement, some of the moisture in the air condenses out onto cool surfaces. This results in a general dampness in a basement. And, this dampness can become a kind of mustiness, since many basements have poor ventilation to circulate fresh air through them.