This is a very interesting subject. There is a surge in people wanting to be more self sufficient – myself included so thank you for the opportunity to do a bit of research. And yes it is a viable option with some considerations.

The name for an underground greenhouse is Walipini – the Aymara Indian word for “place of warmth”.  My search for more information about Walipini shows they are popular in cold climates in the US such as Minnesota and overseas in the Netherlands.  Starting at 4’ below the ground surface the temperature stays at a constant 56-60 degrees Fahrenheit when the temperature at ground level is 10 degrees Fahrenheit. Its is a great way to grow cool weather vegetables such as broccoli, cauliflower, lettuce, spinach - any early spring or late fall crop in the dead of winter.  If you are blessed to live in an area with ample sunshine in the winter you maybe able to expand the type of vegetable crops. I live in West Michigan where it is predominately cloudy all winter long so I would stick witht he cool weather crops in my area.

Before digging and building structures 6-8’ underground I strongly advise contacting your city or county engineer to see if it is permissible to do so. You do not mention if you live on a large parcel of land or in a plotted subdivision. Altering the grade of your property may create issues with stormwater runoff.

Another consideration: depending on what type of soil and how high the water table is may create unforeseen consequences. In my limited construction experience, I have been aware of a situation where the sides of excavated trenches collapsed (with the workers in the trench) due to poor planning. This can be a lifethreating situation. SAFETY FIRST!

I found this article online with an embedded You Tube: https://www.permaculturenews.org/2016/11/28/walipini-greenhouses-diy-tips/

And this one which has additional resource list and YouTube: https://www.permaculturenews.org/2016/11/28/walipini-greenhouses-diy-tips/

This link is to an ebook / manual: http://www.growtest.org/underground-greenhouse-manual/

In your research you may have come across these articles as well. I was surprised not to be able to find much else for a .org or .edu website. Use discretion when reading .com websites as there can be a product they are promoting and the information provided usually is not vetted all that well.

Another option to consider would be a high tunnel hoop house that can be secured to a wooden frame and covered with thick mil plastic over curved cattle panels. It is a very inexpensive way to get a greenhouse type structure. On a cloudy 40 degree day the temperature inside was 70 degrees.  Installing a door at both ends will allow for ventilation in the summer or remove the plastic all together.

Good luck with your project which ever option you decide. Proper planning will eliminate disappointment… The rewards are well worth the effort!