Help to identify this large, slow-growing mushroom?
This fungus is known as a puffball, although that is no longer a taxanomic order. Some of these fungi don't have small openings through which the spores disperse, but they burst open when the spores are ripe. This proces can be clearly seen in your last pictures. The brown stuff below the cracks are the spores. I think when you press it, the brown stuff will come out

Looking at the appearance, I think this is a Calvatia, but as I am unfamilair with the American fungi I can't name a species.