Moisture in basement

[David Hodge]

We are in Colorado (near Denver) and noticed moisture in the basement.  I was originally thinking that the moisture was coming up through the concrete, but I don't believe that is the issue for these reasons... the top of the carpet is damp but when you pull back the carpet, the padding below is dry as well as the concrete.  That leads me to believe the moisture must be coming from the air above, not the concrete below.  In addition, the moisture is evenly spread throughout the entire basement.  If it were a crack in the foundation, it would be concentrated in one spot.  When you walk into the basement you can feel a lot more humidity than above ground.

Has anyone else had this problem?  What could be causing this moisture if it isn't coming up through the ground?  Maybe it's coming in through the windows?  Are basements always more moist than above ground?  Someone recommended we get an air return grill put into the basement to suck the air out and circulate it through the house.  Apparently basements should always have this so the air doesn't sit stagnant.  Is that true?  Any input would be greatly appreciated.