Abstract: An apparatus for modifying an atmosphere for use in a conditioned zone of a structure. The apparatus typically includes an underground air conduit system to take advantage of geothermal conditions to modify the temperature of air and water vapor flowing through the apparatus. A drain is typically provided for removal of water vapor that condenses to liquid in the air conduit. In some embodiments air from the conditioned zone of the structure may be recycled through the apparatus, together with a source of air that originates outside the conditioned zone of the structure. The apparatus may be integrated into other heating and cooling systems as appropriate to further control the air temperature. The apparatus may be combined with a solar heated water heater or “trombe” wall type structure where the heat generation in the winter provides a complete balance for year round stable and livable air temperatures.