Year-Round Gardening in an Underground Greenhouse

How would you like to have fresh vegetables and fruit year round? Sounds pretty good right?  Having an underground greenhouse will keep the temperatures hot in the winter and help prevent overheating in the summer; making it possible to grow your garden vegetables through the cold winter months.
How it Works
We all learned in school that under the earth’s crust is magma which heats the entire sphere. Surprisingly, if you dig down 4 feet, the heating process becomes apparent. For the vast majority of the planet, 4 feet below the surface will stay between 50° to 60°F even if the weather above the ground is 10°F with a cold wind! This phenomenon is called the thermal constant and it’s what the underground greenhouse thrives on.
The original design for an underground greenhouse was invented in Bolivia and was called a walpini, an Aymara Indian word which means “a warm place.” Typically, a walpini is a rectangle shape that is 6-8 feet deep in the ground. The longest area of the rectangle will typically face towards the south (in the Northern Hemisphere) to take advantage of the most sunlight.
Since the walpini is taking advantage of the thermal constant, it requires less help from the sun to heat the greenhouse.
The design of the underground greenhouse isn’t that complicated. It can be as simple as a hole with plastic sheets laid on top. The roof seals in the heat and insulates the area to keep a warm, moist environment for your fruits and vegetables.
more info at - http://www.thereadystore.com/blog/?p=9432
(Thanks Dagny!!)