Ground Source Heat Pump (GHP)
Ground source Heat Pumps are becoming more popular today because of their high efficiencies that they offer compared to traditional heating sources. Heat pumps by definition use the difference in temperature to transfer heat into or out of a building. With ground source the temperature difference is obtained through the thermal heat stored in the soil or water.
Heat pump: noun; "a decvice that uses a compressible refrigerant to transfer heat from one body, as the ground, air, or water, to another body, as a building, with the process being reversible" - dictionary.com