Ether

The ether is a transitional place that links each world to the Wood Between the Worlds. It is accessed from the wood Between the Worlds via a pool while wearing a green ring, and is accessed from each world via a yellow ring. When one is in the Ether, they feel as if they are under water, though they are also able to sometimes see celestial objects of the particular universe such as stars and planets. The ether is likely not a vacuum as people are able to speak to each other and breathe normally while traveling through it. Its only appearance in the series is The Magician's Nephew.