Ethereal Plane
The Ethereal Plane can be considered to be a 'shell' around the Inner Planes, protecting it from creatures outside of mortal comprehension but also holding the intricate lattice of planes together.
Without the Ethereal Plane the Inner Planes would gradually bleed into each other, creating a miasma of realities. The infinite number of versions of the Prime Material Plane would form one shared reality where everything and nothing would try to coexist and the resultant paradox would 'spill out' to the Outer Planes. As the Deities exist to keep the Outer Planes safe from the Inner Planes, the Ethereal Plane was created to 'lock' individual planes and alternate realities in place (although that concept is very difficult to accurately describe for a system that exists in more than 3 dimensions).
Most of the Ethereal Plane is so far beyond the comprehension of mortals that no mortal mind can survive it, but where the Ethereal interacts most strongly with one of the Inner Planes there is enough that is 'recognisable' that mortals can navigate it. This region is often referred to as the 'Border Ethereal'. This means that the Ethereal Plane itself can be traversed through various forms of magic, allowing transportation not only to other planes but alternate realities as well. Some scholars have also theorised that time itself could be travelled along via the Ethereal Plane.
