Even in an area with an extreme climate, the ground maintains a relatively constant temperature. Because of this, a house that is built partly or entirely underground can be more energy-efficient than a home above ground. During the winter, the ground is warmer than the air. During the summer, it is cooler. Any large mass of earth tends to maintain a constant temperature. You can see for yourself how this works by testing how long it takes for a thermometer buried in sand or soil to reach the temperature of surrounding air.