The concept is very simple: a greenhouse that is built into the earth is much better insulated than a conventional greenhouse. A pipe that runs 10m far through the soil (1.5-2m deep) and draws air from the outside is attached to the bottom of the greenhouse, constantly supplying fresh air at a stable temperature. A glass roof catches the sunlight and solar heat is also captured by a large, black water tank that is placed inside the greenhouse and exposed to the sun whenever it is shining. Such a greenhouse is very cheap to build (if old windows and other scrap materials are used) and allows for the extension of the growing season throughout winter as it’s temperature will never drop below a minimum of around 8 degrees celsius.