Ever heard of a walipini? It’s basically an underground greenhouse that utilizes the earth’s natural temperature to keep plants cooler in the summer and warmer in the winter.
The support walls of a walipini are typically built from stone, mud brick, or any other material that is strong and sturdy and can also retain heat in the winter and coolness in the summer.
Taking it a step further, some walpini creators dig a trench on the south side of the greenhouse which allows cool air to drop in and be warmed by the deeper and hotter earth.
Angle your windows 90 degrees so as to allow sunlight to penetrate the greenhouse more efficiently and store heat on days when the sun shines the least.
Be aware of the depth of the water table in your area. If you dig too deep, you could flood out your crop during heavy rains.