A few days ago, my friend Joe (who designs experiences for a living) and I went on a tour of an alternative Bristol. We were taking part in Tom Abba's These Pages Fall Like Ash. I don't know Tom Abba personally, nor have I ever met Neil Gaiman, who had some input into the project, but because we have so many mutual friends this non-acquaintance feels increasingly weird, almost as if we were slipping in and out of interleaved existences - which is very apropos, in fact. The idea of These Pages Fall Like Ash is that you are given a mysterious object (wrapped as all such objects should be in brown paper and string), which contains fragmentary information about Portus Abonae, an even-more waterlogged version of Bristol that occupies the same space as the familiar city, but is only intermittently perceptible. Armed with smartphone/iPad (I own neither but looked over Joe's shoulder) you follow the mildly cryptic clues to places in the city that are narrowcasting - if that's the word - scraps of prose, pictures, and other squintway glimpses of Portus, with a range of a few metres. The project will unfold over the next couple of weeks, with more hot spots coming into operation, so that the experience is lived out in real time - for some values of real. Already, though, the journey has taken us to such mysterious sights as this and this and also to this spot - just a few yards from the place where we celebrated DWJ - another mutual friend of both Abba and Gaiman - exactly a year before.
Anyway, I won't say much more about it now, as the experience isn't finished yet - but it got me to wondering about the whole question of superimposed cities. Obviously Gaiman's got form for this, with Neverwhere. China Mieville's Un Lun Dun also springs to mind. I even dabbled myself, in The Fetch of Mardy Watt. What other examples are there? And which was the earliest? I expect Johns Clute and Grant have something to say about it, but I don't have their book with me.
At any rate, it's perhaps worth distinguishing between your alternative city proper, and the revelation of a hidden aspect to the known city. Something like Charlie Fletcher's Stoneheart, for example, reveals many aspects to London that most of its inhabitants are unaware of, but London remains the location. The same might be said of the Borribles trilogy, or Archer's Goon. I make this distinction - between worlds bleeding into each other, and people bleeding between different aspects of the same world - only to wonder whether it's worth making.