Garden Pests and Diseases: What IS this? Save my tree..!, 1 by yotedog

yotedog wrote:
This thing (a gall???) has appeared at the base of my huge, towering, at-least-60-year-old Willow Oak tree for the last two years. It doesn't appear to do any harm, but I've decided I need to find out what it is, as this tree is only two yards from my house (someone didn't realize how large Willow Oaks get, apparently....) so I value its continued health. I'm guessing its fungal, but not sure if its harmful. The area is quite dry, but does have a fairly deep layer of dried Willow Oak leaves around the base of the tree, and is in complete shade. Any thoughts ?