Geothermal Heat Pumps: What are they? Why are they so efficient?

Geothermal Heat Pumps: What are they? Why are they so efficient?

The federal government gives an energy rebate of 30% for geothermal heat pumps. As far as I know the only other classification that gets this kind of residential subsidy (and is ready for prime time) is solar.  Free money for energy upgrades is a constantly changing...