Immoveable objects.
She'd presumed that they were just an illustrive device - the nemesis of the unstoppable force. It hadn't occurred to her that, actually, they did exist.
Why they existed in a forest was another matter entirely. It wasn't exactly clear (well, the object was, that was why she couldn't see it) why an immoveable object should want to be in a forest. Was there something about forests that made it such a rich environment, suited to objects that resisted force?
Walking around it didn't seem to be an option - immoveable and apparently large. Impossibly large. It was...