This came up in my lawn a few weeks ago. Unlike most mushrooms that pop up with seeming randomness, this one appeared exactly where I would have expected. I cut down a diseased willow tree recently (is there any other kind?!) and ended up leaving quite a few roots in the ground. So the infected roots continued to harbor the fungus, which eventually sent up this thing where the tree used to be.
It’s some sort of what they call a shelf fungus, I think. More typically, these grow out of the branches of infected trees. But since I cut the the tree down, all that was left were underground roots and the only way the fungus could produce its mushroom (basidiocarp, in the scientific vernacular) was up and out of the ground. Anyway, I though it was cool looking. And it was a big one — about 8 inches across.