The folks at Our Little Thing blog have created a wonderful example of a pit greenhouse built with earthbags, PVC pipe, some wood, and plastic cover. This is the second one they have built, having learned many things from the first trial. In the harsh Patagonian climate, they plan to grow many things that are otherwise impossible to grow there, such as avocados and grapes. This seems like a very practical and inexpensive way to provide a large greenhouse space. You can see many more pictures and read more about how they built it on their blog. If you find this interesting, you might also see this recent blog post.