As home heating and cooling costs continue to rise, home builders everywehere are searching for more economical energy sources. Earth-sheltering offers an even better idea: why not save on energy by not requiring it in the first place. The forty designs shown here range from traditional single-family homes to mountain retreats to condominiums. All average 10 percent savings in construction costs and reduce energy consumption by up to 80 percent. These sun-filled houses with their skylights and atriums, decks, greenhouses and well-planned views are just the opposite of the dimly lit cave that the idea of an earth-sheltered house might bring to mind. They represent a new attitude in home building: the desire to work with nature rather than to try to master it.