An earth shelter, also called an earth house, earth bermed house, or underground house, is a structure (usually a house) with earth (soil) against the walls, on the roof, or that is entirely buried underground. Earth acts as thermal mass, making it easier to maintain a steady indoor air temperature and therefore reduces energy costs for heating or cooling. Earth sheltering became relatively popular after the mid-1970s, especially among environmentalists. However, the practice has been around for nearly as long as humans have been constructing their own shelters. Definition "Earth-sheltering is [...] a generic term with the general meaning: building design in which soil plays an integral part." This definition is problematic however, since earth structures (e.g. rammed earth or cob) are not usually considered as earth shelters as they are above ground. "A building can be described as earth-sheltered when it has a thermally significant amount of soil or substrate in contact with its external envelope, where “thermally significant” means making a functional contribution to the thermal effectiveness of the building in question. "Structures built with the use of earth mass against building walls as external thermal mass, which reduces heat loss and maintains a steady indoor air temperature throughout the seasons." "A residence with an earth covering for its roof or walls." "Homes that have been built underground, either partially or completely." "The use of earth cover to moderate and improve living conditions in buildings." History Early history Earth sheltered is one of the oldest forms of building. It is thought that from about 15,000 BC migratory hunters in Europe were using turf and earth to insulate simple round huts that were also sunk into the ground. The use of some form of earth sheltered construction is found across many cultures in history, distributed widely across the world. Normally these examples of cultures using earth sheltered buildings occur without any knowledge of the construction method elsewhere. These structures have many different forms and are referred to by many different names. General terms include pit-house and dugout. One of the oldest examples of berming, dating back some 5,000 years, can be found at Skara Brae in the Orkney Islands off northern Scotland. Another historical example of in-hill earth shelters would be Mesa Verde, in southwest USA. These building are constructed directly onto the ledges and caves on the face of the cliffs. The front wall is built up with