The Ethereal Plane

The Ethereal Plane is a fog-bound plane of half-formed matter that overlaps the Material Plane at every point. In its shallows, the Border Ethereal, a traveller drifts unseen beside the world and sees it only dimly. Further out lies the Deep Ethereal, where vast curtains of moving colour, rippling like an aurora, divide the depths from the borders of each plane it touches; the colour of a curtain tells which plane lies behind it. Pockets of the mist can thicken into demiplanes.

Reaching it. By a spiraling white colour pool in The Astral Sea or a plane shift fork of glass tuned to B.