I have a few large trees in our backyard that were the victims of a fire at some point, long before we bought the property. I don't know if some idiot had built a large bonfire next to the trees or what, but four trees show evidence of some pretty severe burns, some of which are at least twenty feet high. I have been a little concerned about these trees because if one of them were to fall, there is a very good chance that they would land on the house, and there is almost no way that they could avoid taking out our power lines. But they have shown no sign of dying. They leaf out pretty good every year. One of them, however, had a sizable hole near the bottom of the trunk, easily large enough for a squirrel to fit into, although the hole didn't appear to extend upward or downward. The squirrels would store nuts in there sometimes. As early as last summer, the hole was there, because I saw that the squirrels had placed a few nuts in it. I had leaned a piece of pressboard against the tree last fall that some idiot handyman had used in rebuilding our back steps a few years ago. Of course, pressboard isn't known for being able to withstand the elements, so it had deteriorated pretty quickly. By spring, the piece of pressboard had deteriorated completely and was lying on the ground as a pile of mush. Then I noticed that the hole in the tree was gone. It looks like something - and the only thing I can think of is that the squirrels - had used the pressboard mush to plug up the hole in the tree. I can see where the hole was because it's a different color but it has dried hard, like the wood around it. It wasn't a small hole either; it was at least three inches in diameter.
Some poor squirrel probably hid his stash of winter nuts in there and sealed it up good and tight. Just wait until he comes back to retrieve his stockpile and finds that it has cemented itself together !