Lord Zack's Planescape
the Ethereal Plane
The ether is the force that binds all matter together. The ethereal plane is the fundamental plane that connects all others together. Ether is sometimes considered the fifth element.
The ethereal plane has border regions that connect to the planes that it touches. This border region is slightly out of phase with the plane. As you get more and more out of phase you enter the deep ethereal. From here you can find you’re way to other planes connected by the ethereal.
Rumors tell of an ancient, eldritch deity that can be found in the center of the ethereal plane, a horrible being of chaos. These rumors are unsubstantiated, of course and some debate whether or not there is a center to the ethereal plane at all.