Thermal Mass is a given materials ability to absorb and store heat energy from the sun. If designed effectively a building may reduce the need for heating and cooling methods to 0. Different materials have different thermal mass, and are always responding to the heat of the sun, or coolness of the air. However beyond 4 ft below ground level, the earths temperature becomes regulated at a steady 58 degrees. Because heat always travels to the cooler areas, a building wrapped in the earth, will receive warmth when cooler than 58 degrees, and will have warmth absorbed from it when above 58 degrees. This makes the earth 4 ft and beyond below ground level the most effective temperature system for any climate.
http://sustainabilityworkshop.autodesk.com/buildings/thermal-mass