The earth has a much more constant temperature than the air. It might be 100 degrees in summer and -40 degrees in winter where you live. But a few feet down, the earth will always be a steady temperature.
For every 100 feet you go down into the earth, the temperature rises 1.5 degrees. So if you were to dig down to a depth of 600 feet, assuming the temperature a few feet below ground is 60 degrees, the deeper rock would have a temperature of
69 degrees.
