Here is what appears to be a highly successful example of a deep earthen greenhouse that uses solar, thermal mass, and earth tubes to grow citrus and other fruits economically year-round in Nebraska.  

The greenhouses are economically and ecologically self-sustaining as they're organic and the costs of conventional heating, cooling, and long-distance transportation have been eliminated.

What makes this even more special is the builder/farmer is an 86-year old "retired" postal worker who still does his own construction. 

Priceless!

http://greenhouseinthesnow.com/index.html