Five hidden worlds just below London
1. You yourself may have encountered pigeons which seem to have only one leg, or perhaps one and a half legs, or even some other fraction of legs. This is an illusion. These pigeons are abundantly supplied with leg. Some even have three. London being what it is, as countless authors can tell you, sometimes has rains from the sky of teensy-tiny portals to grubby underworld dimensions. These pigeons have got their feet stuck in them. Even now their feet are having excitingly gothic adventures without them. This is why pigeons go into bars and cry.
2. Similarly, you yourself may have had the sort of incident in London where your pie and a squirrel come into closer contact than you as a pie-eater would like. Would you be surprised to learn that this is also due to a murkily sinister alternative dimension lurking just below London? I would not be. These squirrels are in exile and are constantly on the lookout for a route home. Their original portal was cunningly designed as a rotating pie; press on one end, and you are flipped down into the raucous depths. Is it any wonder that they seek to spin your pastries?
3. Have you seen the sort of genteel little city square whose bounds are locked and triple-locked against the entry of plebs like you? This too is an illusion, and a cunning one. Step past the fence and it all becomes clear: what seems to be a square is actually a reflecting pool of cunningly-fashioned ghoststuff leading into yet another moist little underLondon. There are many of these in London, because there are.
4. London being, as I believe I have mentioned, a hive of underdimensions over overdimensions beside portals through portals; that said, you can imagine the challenges to build an underground railway through the place. Tubular engineers must take an oath never to reveal the sinister doorways that have yawned in their general direction. In fact, there are many gaps in the network where tube trains simply leap across chunks of other worlds or, in one notable example on the Victoria line, a small segment of translocated deep space. Sometimes one may even hear the whistling chittering of those things that lurk beyond. But usually not if you have headphones in.
5. Then again, there is that other theory: that we are already all living in a London below some other London and its extended outworld. Scientists who have looked into the matter say that this is by far the most likely option. Indeed it may well be that we are all living in a London below some other London that is itself below an even Londonier London, and so on all the way up. Who knows what the top of that stack looks like. Perhaps it connects again to the bottom, leaving us with no London at all other than the ones we made along the way.