Here are plans to make an Earth-sheltered Greenhouse from a man in Idaho. He actually built it into the side of a hill to help moderate the temperatures inside. It looks like he dug it all out by hand. (Click on Image Gallery for close-ups.) He must have wanted vegetables badly! I’m pretty sure if we tried something like that on our land, the greenhouse would fill up with water. We have a pretty high water table when there isn’t a drought. If this doesn’t seem like it would work for your situation, check out some of the other season-extending books in my Amazon Store.
Thanks to Primrozie for the link.