Subject: At what depth in the earth can you reach a constant, sustained temperature?

My intent is to be able to reach a depth of twenty or so feet, and place a 
method of air flow and attatch it to my central air. My question is really to 
determine at what depth I need to place it. I am hoping that it is with my 
means to achieve this by hand. Thank you for your response. P.S. What is the 
constant, sustained temperature that I could reliably achieve at twenty or so 
feet..? Thanks again.

Re: At what depth in the earth can you reach a constant, sustained temperature?