Geothermal

taking advantage of energy differential between the surface and the near-surface

Malcolm Gladwell's father provides some experience

Related: Geo Exchange: The engineering and scientific communities prefer the terms "geoexchange" or "ground source Heat Pump-s" to avoid confusion with traditional geothermal power, which uses a high temperature heat source to generate electricity. http://en.wikipedia.org/wiki/Geothermal_heat_pump