A MetaPlane is a distinct world or universe accessable through the AstralPlane.

*more to follow...*

Well known (to advanced mages and magic theorists anyway) Metaplanes in [Boston] inclue the four Elemental Metaplanes: [Earth], [Air], [Water] and [Fire]. As well as the Nature Metaplanes inhabited by the [Totem]s: [The realm of man], [The Waters], [The Sky] and [The Land].

Other magical traditions may know of, and have access to other relevent Metaplanes. 

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In [Camrish], these are more usually referred to as the [NetherWorld]s.  Cascade and Arcadia are noted examples.

<hr> Getting to the [NetherWorld]s involves either taking a physical gate, or interceding with the [Dweller on the Threshold] to gate you through.