Ethereal Plane

From publisher blurb:

Enter the Ethereal Plane
The Ethereal Plane is a misty, fog-bound dimension that is sometimes described as a great ocean. Its shores, the Border Ethereal, overlap the Material Plane and the Inner Planes so that every location on those planes has a corresponding location on the Ethereal Plane.

Certain creatures can see into the Border Ethereal, and the see invisibility and true seeing spells grant that ability. Some magical effects also extend from the Material Plane into the Border Ethereal, particularly effects that use force energy such as forcecage and wall of force. The depths of the plane, the Deep Ethereal, are a region of swirling mists and colorful fogs.