How to Build an Underground Greenhouse: A Backyard Project That Extends Your Growing Season The image above shows a powerful transformation in three simple stages: excavation, framing, and completion. It’s the perfect example of an underground greenhouse—sometimes called a sunken greenhouse or a walipini—designed to use the earth’s natural insulation to protect plants and extend the growing season. This is one of the most practical DIY projects for gardeners who want to grow more food with less