And cooler in the summer I'd bet.
Mine isn't in the ground, but that sounds like an interesting idea. Wonder if the humidity would be too high. You could probably get the same effect by building it into a hill, or building a hill onto it. Plenty of people are doing this with their own houses...
Maryam, that looks like really nice flooring. Part of my duck house has a grate, but I'm using just welded wire, it doesn't look as comfy as yours. Tell me, where did you get that dog run stuff?