One of the most interesting eco-friendly systems Stockholm Arlanda Airport uses is their unique heating and cooling system for their hangar, terminals, and other buildings on the airfield. There innovative system uses a series of wells, linked to a large underground aquifer. The water from the underground source is plumbed up and into the facilities air system, which controls the temperature of the air coming from the vents. In the summertime, the underground water remains cooler than the surface. That allows the terminals to be cooled off without using extra energy that an air conditioner would require. Then, in the winter months, the underground water remains warmer than the surface. The water is then plumbed to a control/heating unit, which uses biofuel to heat the water to a temperature appropriate for warming up the buildings.