Then he saw that he was sinking down towards the mottled surface of the giant world, and that another of the rectangular chasms yawned immediately below. The empty sky closed around him, the clock crawled to rest, and once again, his pod was falling between infinite ebon walls, towards another distant patch of stars. But now he was sure that he was not returning to the Solar System, and in a flash of insight that might have been wholly spurious, he knew what this thing must surely be.
It was some kind of cosmic switching device, routing the traffic of the stars through unimaginable dimensions of space and time. He was passing through a Grand Central Station of the galaxy.
Judging from the cut of you, you're more of a world traveller. Well, you've come to the right place! This underground realm, like the great British rail system, is the very model of efficiency. Agartha's thoroughfares sprout from the Tree of Life, and connect back to the surface. Distance and time bend in here. Why, you can cross the globe in a brisk walk!
—The Stationmaster, The Secret World
No one knows where this transitional world is. It may be an alternate plane of existence, or a place between planes, interleaved between them like a flower pressed between pages, or a master plane that contains all planes the spine that gathers the pages and binds them together. To the naked eye it looks like a deserted city, an endless series of empty stone squares, but it serves as a kind of multidimensional switchboard. In the center of each square is a fountain. Instead of water, the fountains run with a jet-black liquid, as black as oil, or ink. Step into one of them, the stories goes, and you'll be transported to another universe. There are hundreds of different squares, possibly an infinite number, and a corresponding number of alternate universes. The bunnies call this place the Neitherlands because it's neither here nor there or sometimes just the City.