The Transitive Planes: the Ethereal Plane and the Astral Sea

The Ethereal Plane and the Astral Plane are called the Transitive Planes. They are mostly featureless realms that serve primarily as ways to travel from one plane to another. Spells such as etherealness and astral projection allow characters to enter these planes and traverse them to reach the planes beyond. The Ethereal Plane is a misty, fog-bound dimension that is sometimes described as a great ocean. Its shores, called 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. The Ethereal Plane also forms the 'wall' (or 'gap') between planes. Certain creatures can see into the Border Ethereal, and the see invisibility and true seeing spell 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.   The Astral Plane is the realm of thought and dream, where visitors travel as disembodied souls. It is a great, silvery sea, the same above and below, with swirling wisps of white and gray streaking among motes of light resembling distant stars. Erratic whirlpools of color flicker in midair like spinning coins. Occasional bits and even habitable islands of solid matter can be found here (these are notably the homes of the Gith races), but most of the Astral Plane is an endless, open domain. The Astral Plane is the location of most of the natural or artificial demiplanes, and is mingled with the divine energy of the saviour-titan Hestia to form the Aether, the border between the Prime Material Plane and the Celestial Plane.   Source: https://www.dndbeyond.com/sources/basic-rules/appendix-d-the-planes-of-existence#TransitivePlanes