No, I don't think there are... but there's this sense it could be this whole world that is out of sight of humans. Mind you, it's got its own elaborate mythology and all. I lost track somewhere after about the first seven, which I read when I was a teen, and he was still going with the things last time I looked. (How many times can you write the mice and the Badgers fought the Nasty Weasel/Fox/Ferret and won? 8-o)