The London subterraneans are real troglodytes, born and bred down below and seldom if ever coming to the surface. I have never seen one of these subterraneans, nor so much as met a person who has seen one. Much the same is true for all the things that live down below in subterranean London beside them. They are planning a James Herbert-style blitzkrieg and the victims of it will be you and me. There are, however, no alligators.
Legends are a kind of imagined history.
And so to the London Subterraneans. I can't recall when or where I first heard about them and that's bad form for somebody who's supposed to make notes on place and provenance as a matter of routine. All I know is that I seem to have been aware of the troglodytes for a very long time and that everyone else I've found who is aware of the same legend is equally vague about where they came by it. It is fairly consistent, as such legends go. The underground race evolved from a small group of humans who fled below out of desperation - outcasts from society, perhaps, who had nowhere else to go.