Cool link!Make sure you don't get moldy bedding and feed down there- that can kill your chickens.
Saw this awhile back:
https://www.backyardchickens.com/a/oblio13s-underground-chicken-coop
Wonder how he did for moisture buildup or water drainage....
I love the idea of an underground coop but would still think it's best chace of working would depend on soil factors and terrain for drainage issues.