I live in Texas, where it is hot, and I'm faced with the challenge of keeping fermenting beer cool. I don't have a cellar because those are illegal in Texas, or so one would think since nobody has them. I can't just use my house because we keep the temperature in the high 70s or low 80s to conserve energy. The actual correct solution is to buy a chest freezer or refrigerator for the purpose, but at the moment I use a styrofoam chamber and gallon jugs of ice. It works ok.
Someone at work suggested that I could burrow down in my backyard and install a coolant loop under my shed. Since the ground stays cool, I could circulate water through the ground and obtain a year-round fermentable temperature that would neither freeze nor get hot. Everyone knows caves stay 56 degrees F year-round, after all.
So, how far down DOES one have to burrow in order to achieve stable, cool, cave-like temperatures? I have a feeling it's a bit more than 6 feet.