Thanks for the link yes I can see the pictures better there now.
Cool idea I found a clip from Montana where they build the greenhouse 6ft down and they claim it makes it go from zone 4 to zone 8 without any heating:
https://youtu.be/ZO4yO0Nxv9A
Keep us posted on your project so we can learn from it. I'm moving to zone 7 this year and want to build some kind of greenhouse so I can keep growing what I do here in 9b.
Digging it down a few feet might be helpful as long as the area doesn't have a high water table. I wonder if 2 feet would suffice for a south facing passive solar greenhouse (hopefully built lean to style on the house south facing wall) to keep above freezing in 7a/b
5-6 feet is a lot to dig for a large greenhouse (like in the video)