I’ll be the first to admit that the below photos aren’t that great in terms of providing a visual example, but it’s a cool hollow on this tree so there’s that reason why I want to share it as well.
Anyway, as was briefly touched upon in my post yesterday on obstacles to urban tree planting, one concern residents may have about trees is that they can be used to cache items (potentially drugs or other contraband). By a similar token, hollows can be used as bins. In the below images we can observe exactly this, albeit only on a small level. In terms of how the hollow came to be, I suspect there was a large branch tear-out wound that, in time, rotted away. The slight tail in the wound suggests this may indeed be the case, and frankly I don’t know what else could have caused it!
Arboriculture 