[Tilted Irish Kilt]

texas_mrs :" If we do build an underground greenhouse,.."

What do you mean by " If we do build an underground greenhouse ..." ??
Obviously , you have done some significant research already , as have I, back in my "hippy-dippy, bell-bottom " years (decades ago).
When looking at "tubebender"'s greenhouse photos , I was reminded of several other optimal items for a greenhouse :
Thermometer : preferably with a battery operated alarm system should the greenhouse get too warm/cold ,
or when you aren't immediately available, a thermocouple operated automatic vent (depending on the size of the greenhouse). A convenient place for Whiteboard/ supplies list for items that you are running short on ; that way you won't run out.
A radio for music / entertainment, to make the greenhouse a relaxing place to be ,according to a book named :" The Secret Life of Plants" ,
but also because plants seem to respond according to our emotions when near them.

[texas_mrs]

Obviously , you have done some significant research already , as have I,

Yes, for several years now I have been fantasizing about it and so have done a lot of looking around on the net. I'm fascinated about the idea of a 'chimney' for ventilation and cooling and am considering that method over the ground tubes. There are so many things to consider - but hopefully I will have it all figured out before we begin.

[tubebender]

You're taking the fun out of gardening TIK. I have "tended a garden" since 1939 when I was 6 years old and our current popup works well and my failures have been few and far between and and are far from a commercial operation like the ten acres under glass of Sun Valley Floral Farms in a town nearby where minutia rules the day

[tubebender]

This model was the talk of the town for 25 years and it had a few of the amenities you suggested but old age and wood rot claimed it. I will never forget the day I walked out on the deck to look at our pride and joy and it had collapsed…