One of the main principles involves embedding the greenhouse in the earth to take advantage of the earth’s constant temperature, to store the solar energy collected during the day. Water barrels can also be used to store the thermal heat and carry it through the night or cloudy days (which are not as cold). Water is a much better thermal mass storage mechanism than soil. This is perfect for us preppers and hardcore gardeners, I would also take advantage of this and maybe install an aquaponics system in this space too or a small fishery.

More at Source: How To Build An Underground Greenhouse And Have Food All Year Round – SHTF & Prepping Central