I don't remember where I read it, many years ago, but I read a story about an underground greenhouse that incorporated on at least 2 sides, deep pits that were filled with leaves every year. There was some type of mesh opening that allowed airflow. The act of the leaves decaying added warmth to the greenhouse. I've often wanted to try to duplicate that, but never had that ability. But it seems to make sense in my mind. I know there have been studies about compost piles. So similar, but less stinky.