Optimal Green: Modern Earth Sheltering
Earth sheltering is an effective way to regulate temperature in a home. It works well in almost any climate. Just below the frost line, the earth stays a fairly constant temperature: 50 degrees F, plus or minus a few degrees, depending on where you live. The surrounding soil provides natural insulation, making these houses inexpensive to heat and cool. The best location for an earth sheltered house is on a well-drained hillside. Windows facing the south or an overhead skylight will fill the interior with sunshine. In the winter, earth-sheltered homes require very little additional heat to warm them. In the summer, the interior stays cool — while it may be 100F outside, the house will stay at comfortable 70F, thanks to it's earthen wrap.
There is also an additional advantage: earth shelters absorb rain water. This means less water runs off the building, and get all the benefits of having plants grow in this micro-habitat: e.g. temperature and humidity regulation.
If it's just the walls that are sheltered with earth, it's called "earth berming". When the roof is covered, it's called or a "green roof" or a "living roof". When the entire house is covered, it's an "underground home".
See also: Alex Michaelis' Underground Green House