and then backfilled with earth. An experimental construction design from the 1980s of Japan, coined 'Alice City', proposed to use a wide and deep cylindrical shaft sunk into the earth, with a domed skylight roof. Artificial caves can be constructed by making a tunnel into the earth. Building greenhouses underground has also been speculated about. Schools, commercial centres, government buildings and other buildings could be built underground. Applications Active and passive solar Earth sheltering is often combined with solar heating systems. Most commonly, the utilization of passive solar design techniques is used in earth shelters. In most of the Northern Hemisphere, a south-facing structure with the north, east, and west sides covered with earth is the most effective application for passive solar systems. A large double glazed window, triple glazed, spanning most of the length of the south wall is critical for solar heat gain. It is helpful to accompany the window with insulated drapes to protect against heat loss at night. Also, during the summer months, providing an overhang, or some sort of shading device, is used to block out an excess solar gain. Passive annual heat storage Passive annual heat storage is a building concept theorized to create a year-round constant temperature in an earth shelter by means of direct gain passive solar heating and a thermal battery effect lasting several months. It is claimed that an earth shelter designed according to these principles would store the sun's heat in the summer and release it slowly over the winter months without need for other forms of heating. This method was first described by inventor and physicist John Hait in his 1983 book. The main component of is an insulated and waterproof "umbrella" which extends out from the earth shelter for several meters in all directions. Hence the term "umbrella house". The earth under this umbrella is kept warm and dry relative to surrounding earth, which is subject to constant daily and seasonal temperature changes. This creates a large heat storage area of earth, effectively a huge thermal mass. Heat is gained via passive solar in the earth shelter and transferred to the surrounding earth by conduction. Thus, when the temperature in the earth shelter dips below the temperature in the surrounding earth, heat will return to the earth shelter. After a time, a stable temperature is reached which is an average of annual heat changes in the external