An earth tube is a tube that is buried in the ground which heats or cools air moving inside it. Earth tubes may a be a closed circuit where they take air from inside a structure, circulate it in an underground loop before returning it to the structure, or an open circuit which takes air from outside and brings it into the structure, or counterflow open circuit when air is brought in and indoor air is taken out in sepearate earth tubes. The air can be moved passively with convection or actively with fans (air blowers). Whether the air is heated or cooled depends on the outside air temperature relative to the indoor temperature, and importantly the temperature of the soil at the edepth the tube is buried. This article focuses mainly on open systems.