The Underground Greenhouse)

http://www.bensoninstitute.org/Publication/Manuals/Walipini.pdf

The Walipini, in simplest terms, is a rectangular hole in the ground 6 ‛ to 8’ deep covered by plastic sheeting. The longest area of the rectangle faces the winter sun -- to the north in the Southern Hemisphere and to the south in the Northern Hemisphere. A thick wall of rammed earth at the back of the building and a much lower wall at the front provide the needed angle for the plastic sheet roof. This roof seals the hole, provides an insulating airspace between the two layers of plastic (a sheet on the top and another on the bottom of the roof/poles) and allows the suns rays to penetrate creating a warm, stable environment for plant growth.