No idea what the subsidereum is.
Michio Kaku has theorized super-string dimensions up to 10. This predicates a GUT that would include all kinds of neat sub and super-space phenomena that could be accomplished manipulating quantum field structures. Everything from "folding space" to creating hyper-dense packets of gravity. Not too mention the existance of completely "other" Universes entire.
This isn't just "sci-fi", this is as real now as Lasers and anti-matter were in the 30's.