"Earth-sheltered" homes are nestled into the earth, either totally or in part. The advantages of doing this are considerable. About six feet under the earth, you will find that the temperature varies by only a few degrees year round. While this temperature might be too cool for general living comfort, you can use the stability of the earth's temperature to moderate the thermal fluctuations of the house. This means that it will take much less energy to either heat or cool the house. If you dig into a south-facing hillside to build, or berm the north part of the house with soil, you can take advantage of this. The part of the house that is underground needs to be well insulated, or the earth will continually suck warmth out of the house.
Another advantage of earth-sheltering is that aesthetically the house can fit easily into the landscape. The profile of the house can be much less imposing. Designs that employ living roofs actually return much of the footprint of the house back to nature, since the roof can be planted with grasses or flowers.
Earthships are specific kinds of earth-sheltered designs, pioneered by Michael Reynolds, which utilize recycled tires for creating substantially bermed walls. His concepts also often employ water catchment from the roof, gray water reuse, compost toilets, and other innovative conservation measures.