Went to a friend's place yesterday and checked out his 'Walapini' (below ground) greenhouse. Pretty neat. With only a clear poly-tarp roof, the temperature in it was stable enough to have kept their lettuce, greens and a couple of tomatoes going all winter in addition to some potatoes & carrots. I've been thinking about something similar for a while now but never got to it.....too many other irons in the fire. Theirs is apx. 10 ft. wide x 25 ft. long x 8 ft. deep. Even as dry as it is here, they only need to water it about every 3 weeks. When the moisture evaporates out of the ground, it condenses on the roof & falls back down on the plants. I like that.