buildings, sometimes whole communities. This technique, long practiced throughout the world in locations such as Reykjavík, Iceland; Boise, Idaho; and Klamath Falls, Oregon; is known as district heating. In Europe alone 280 geothermal district heating plants were in operation in 2016 according to the European Geothermal Energy Council (EGEC) with a total capacity of approximately 4.9 GWth. Extraction Some parts of the world, including substantial portions of the western USA, are underlain by relatively shallow geothermal resources. Similar conditions exist in Iceland, parts of Japan, and other geothermal hot spots around the world. In these areas, water or steam may be captured from natural hot springs and piped directly into radiators or heat exchangers. Alternatively, the heat may come from waste heat supplied by co-generation from a geothermal electrical plant or from deep wells into hot aquifers. Direct geothermal heating is far more efficient than geothermal electricity generation and has less demanding temperature requirements, so it is viable over a large geographical range. If the shallow ground is hot but dry, air or water may be circulated through earth tubes or downhole heat exchangers which act as heat exchangers with the ground. Steam under pressure from deep geothermal resources is also used to generate electricity from geothermal power. The Iceland Deep Drilling Project struck a pocket of magma at 2,100m. A cemented steelcase was constructed in the hole with a perforation at the bottom close to the magma. The high temperatures and pressure of the magma steam were used to generate 36MW of electricity, making IDDP-1 the world's first magma-enhanced geothermal system. In areas where the shallow ground is too cold to provide comfort directly, it is still warmer than the winter air. The thermal inertia of the shallow ground retains solar energy accumulated in the summertime, and seasonal variations in ground temperature disappear completely below 10m of depth. That heat can be extracted with a geothermal heat pump more efficiently than it can be generated by conventional furnaces. Geothermal heat pumps are economically viable essentially anywhere in the world. In theory, geothermal energy (usually cooling) can also be extracted from existing infrastructure, such as municipal water pipes. Ground-source heat pumps In regions without any high temperature geothermal resources, a ground-source heat pump (GSHP) can provide space heating and space cooling. Like a refrigerator or air conditioner, these systems use a heat pump to force the transfer of heat from