Astral Sea

The Astral could be reached from almost any point in a Prime Material plane or first layer of any Outer plane by spell, psionic ability, or device. It was described as a barren place of other-dimensional nothingness extending in all directions. What little solid substance that floated in the bright, gray void was typically chunks of matter broken off from their original plane. The Astral had no gravity but objects did retain their mass so you could throw small items or push off from large objects to move in the weightless environment.

Geography

Upon entering the Astral plane, travelers saw a silvery color pool nearby—a portal to the location on the Prime Material plane from which they originated. Astral projecting travelers saw their silver cord leading back to this pool. Color pools appeared as two-dimensional circles about 10‒60 ft (3‒18 m) in diameter and only visible from one side unless they had some way to detect invisible objects. Pools of different colors were portals to the different Outer planes. Each Outer plane had its own unique color, but the traveler's home portal was always a metallic silver, rippling like mercury in a pan. Color pools could be used to view the destination plane before stepping through by mentally concentrating on the nearby pool until it became transparent. A viewer could also move (with some limitations) the portal's viewpoint by concentration. Astral projecting travelers formed a new physical body (with silver cord attached) when they stepped through a color pool to their destination plane. The new body was formed out of local materials so the greater the similarity between one's home plane and the destination, the more one's new body looked like the original one.
It was considered to be boundless.

Fauna & Flora

All manner of fiends, celestials, slaadi and other planars used the Astral as shortcuts to their business elsewhere, but both astral devas and shedu patrolled the plane regularly to keep evil in check. Despite their connection to the Abyss, bebiliths were thought to be at home on the Astral plane.

The existence of true natives was doubtful. The githyanki had been a dominating presence for long ages, ruling from innumerable fortresses. Spectral hounds were usually found with githyanki communities serving as guard animals.
Astral searchers, astral streakers, astral whales, the mighty astral dragons, and the tiny kodragons inhabit the Silver Void. The dreaded astral dreadnoughts are believed by some to be a manifestation of the Astral Plane itself, while others point to their indefinitely long tails as an indicator for an origin elsewhere.
Brain collectors, dhours, and foo creatures can all be found here.

Type

Planar Sphere/Grouping