Ethereal Plane

The Ethereal Plane isn't really a realm of its own. It overlaps the Foundation and one can see into that realm of mortals from the Ethereal. Everything seems desaturated, blurry and flickers as if flames in a wind. Some have likened it to not so much try to see the world through smoke as to trying to see a world made out of only smoke.
Some things that no longer exist on the Foundation can be represented here if enough feelings are associated with it. But whatever those feelings were, the remnant is now in a state of decay where it can only long for what it once was. A parking lot might have the rotting memory of a playground that once stood there. The swings brow slightly in the wind and far away echos of children laughing come out of nowhere. On the ground is the blood of the child who died there.
Amongst many other things, the Ethereal is the natural home for the spectral undead who can't let go of watching the lands of the living and aren't pulled to one of the other worlds, such as the Land of the Dead. From here, they can reach out, but never grasp, their loved ones. They can sit next to someone during a nice dinner, but can't eat the food or partake in the conversation. They can't even hear the actual words spoken through the filter between the worlds.
It is possible to interact between the worlds to some extent. Anything on the Foundation is semi-solid in the Ethereal, though it is possible to push through it. Actually affecting anything beyond the barrier requires very special abilities. But those who can do so might reach for minds or souls rather than objects.

Fauna & Flora

Few things grow here, and the things that do are actually the remnants of souls where only enough to form a plant remains. The exception is that there are some off-shoots of the Thicket that have taken hold in a few areas.

Type

Dimensional plane