Ethereal Plane

The ethereal plane is where forgotten souls phase into. It is easy to accidentally phase into it, but significantly harder to keep an alive (in body/memory) being there. The entrance of an alive body into the ethereal plane was actually found to be the transfer of the soul into an ethereal body, while the true body is held in Stasis in Limbo. This is because the ethereal plane us knowingly fragile, it cannot hold the pressure of a material body. If anything were to pierce the ethereal plane, it would drain into the astral sea immediately.
The ethereal plane acts as the mucus between each plane, diffusing out the forgotten and unused Immateria into Recreation, in a similar process as Materia Recycling in the astral sea.