There are quite a few reasons to consider building underground. Not only are the houses generally much more energy efficient than building above ground but if you look into the much lauded $50 and Up Underground House Book you can figure out ways to cut your building costs to half what you would pay for a normal construction (or an extraordinary construction like that of Bill Gates). The book also promises to help you build "the most sunshine-filled, best ventilated and driest underground house of all".
The home pictured above is Cumbria’s First Earth-Sheltered House built in an old quarry site and finished in 2003 after 6 months of construction. It doesn't need a heating system and being underground means that the house disappears into the landscape "making it unobtrusive visually as well as ecologically."