This is a must see if your ever in New Mexico. An earthship house is made up of rammed earth and tires. The tires are stacked (staggered) like bricks. Each tire has earth pounded into them until firmly packed. Once the tires are packed, they are very difficult to move and form quite a dense wall. The walls are load bearing and provide thermal mass which is an important attribute to any energy efficient house. Thermal mass stores heat and releases the heat slowly. This keeps indoor temperature constant while outside temperatures fluctuate. Once the walls are in place, the walls are quite often plastered over and appear very similar to an adobe style house.