In the middle of the woods there’s a large maple tree. It seems to be in perfect health, and the leaves that fell from this tree all seemed healthy, too. But the other day I noticed an oddly-shaped white thing under the tree; it seems to be a maple leaf that has been attacked by and covered with some weird thing.

Is it a fungus? I’ve never seen anything like this before. Have you? Do you know what it is? (Click on any of these images to see a larger version.)

Update: mystery solved! It’s pulmonaria.