Cave house, in Festus, Mo., is in a 17,000-square-foot gouge left by a 1930s sandstone mine. Close to the Mississippi River, the home is 45 feet under a forest. The owner chose to build in the cave because of the serenity and privacy he and his wife felt on the first day they visited the site. The front of the three-bedroom home is constructed from glass doors and used materials bought from a local store. Insulation sealant keeps the interiors 65 to 70 degrees year-round.