Underground homes are a term used to describe a house built into the ground or slope of a hill, or which has most or all exterior surfaces covered with earth. Because its earth insulation serves as a thermal mass, it retains warmth in winter and remains cool in summer. Utility costs are minimised, and it can be heated with solar panels. Research indicates that underground houses use about half the building materials required for a conventional above-ground house. For the most part, they are fire-resistant and impervious to tornado and hurricane winds.