Years ago while building a new house, the owner hit a cavern while drilling a well. He noticed that the air coming out the drill hole was a constant temp between 72 and 74 degrees. He installed duct-work and a blower to drive the air into his home and the small electric bill is less than $20 per month.
W Bush's house in Crawford has something similar. I read it in a comparison between W's environmentally friendly ranch house and Al Gore's sprawling energy guzzling mansion.