Geothermal energy is readily available virtually everywhere on the planet, and can often be completely passive. It's a "direct use" energy, in that heat from below the surface can be transferred to heat homes, businesses, pools and spas without the need to convert it to electricity. In fact, geothermal heating can be and often is completely passive, although users in some more extreme climates or applications may choose to use electricity-based equipment to move or concentrate heat from underground. In these cases, fans, electronic thermostats, residential heat pumps, geothermal heat pumps, and various other equipment may be used. Nonetheless, it is absolutely possible to operate a geothermal heating system off the grid.
The specific choice depends on your goals, your climate, and the design of your system. Geothermal energy comes from the heat stored within the Earth's mantle - a layer of superheated rock nearly 2,000 miles thick. The heat from this mantle radiates outward through the planet's thin crust, and can be easily detected in volcanic regions or where thermal springs are found. Humans have taken advantage of geothermal energy since time immemorial, through structures as simple as root cellars. Root cellars were used to store food and vegetables, since the temperatures there never fell below freezing and were always cooler than the hottest days of summer. In fact, thanks to the heat rising from below the surface, soil about 8' underground in most parts of the world remain a steady 55 degrees Fahrenheit - perfect root cellar weather. Accessing this well of warmth can be as simple as laying lengths of 6" pipes underground and pumping air through them. The air is cold at the start of the pipeway and has warmed up by the time it exits.
