Put this phrase into Google:

'in ground greenhouse'

Be astounded at the results :)

I like designing floorplans and such. I want an in ground also, for citrus and such. If you dig into the hill side have the 'side walls' be at no more than a 45% angle from the back wall ( To allow more sun). you will definantly need something ro keep the dirt collapsing if you want them vertical. I wouls sugest chicken wire. The wood you use to flush against both the wire and dirt wall ( this can later be stucko'd with concrete, or you can plant grass/chia/etc to keep it from degrading and looking like chicken wire on dirt lol).

In the process of finishing my pit green house. 24'x16' x8' deep. would love to discuss with someone who has one. Growing a little of everything to see what will grow best. At 29 degrees my green house has not been below 65 degrees. Using 55 gallon drums of water to supplement earths heat.

What about using straw bales for extra insulation?

We stack square bales up like bricks around the foundation of our greenhouse. It protects the plants from the bitter cold for the few weeks that it is really cold here. I'm in north east Texas so it doesn't stay cold for long. This system works great for us.

After the temps rise, we break down the bales to use as mulch in the garden. Then after the season is over, we clean up the old mulch and it gets tossed in our compost bin for final decomposition.