Why is my basement so humid? How can I dry it once and for all?

Okay, I know that basements do get humid and stinky. But, some basements don't, especially new ones. I have a townhouse with central A/C and a walkout basement that is fully open on one side. The side that is under ground is dry, and the air on the side that opens to the back yard is so moist that a piece of paper will moisten up and not stand up straight in your hand after a couple of minutes. The doors/windows seal completely. I had it fully inspected for cracks and leaks, and its fine. I had it inspected for mold...also fine. It's just moist and stinky...for what seems like no apparent reason. I bought a dehumidifier and am now emptying about 3 or 4 gallons of water per day out of it and am paying energy bills for this device that I wish I didn't have to attend to to begin with. The only reason I run the dehumidifier is because if I don't, the moist room starts to stink. Where is all this water coming from and how can I dry it permanently?