Geothermal heating has been use since the time of the Roman Empire as a way of heating buildings and spas by utilizing sources of hot water and steam that exist near the Earth's surface. The ultimate source of geothermal energy is believed to be from the radioactive decay that occurs deep within the Earth’s crust. Some areas, including substantial portions of many western states (U.S.), are underlain by relatively shallow geothermal resources. Similar conditions exist in Iceland, parts of Japan, and other geothermal hot spots around the world. These resources can be classified as low temperature (less than 90°C or 194°F), moderate temperature (90°C - 150°C or 194 - 302°F), and high temperature (greater than 150°C or 302°F). Where such geothermal resources are available, it is possible to distribute hot water or steam to multiple buildings. This technique, long practiced throughout the world in locations such as Reykjavik, Iceland, Boise, Idaho,, and Klamath Falls, Oregon is known as geothermal district heating.