"Taxon. We don't know exactly where it's located. It's an enormous cavern on some planet in an alternate dimension. At least, we assume it's an alternate dimension." Given the number of people here who were from clashing realities - the differences between the kind of vampires he and Spike are and Godric and Eric are, for one - makes the alternate dimension assumption the most plausible. Not like they haven't experienced that before prior to this place.