The Astral Sea

The Astral Sea is the realm of thought and dream, where visitors travel as disembodied souls to reach the planes of the divine and demonic. 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 of solid matter can be found here, but most of the Astral Plane is an endless, open domain.
The Githyanki sail the Astral Sea in ships of their own design. Travelers on the Astral Sea must envision their destination, often one of the Outer Planes. Their ship then begins to head towards their destination. The Githyanki know the ways to each Outer Plane, and are able to reach any destination relatively quickly (weeks instead of months). The Githyanki Empire exists on the islands of matter that float through the Astral Sea.
Other travelers report that the Astral Plane is shaped like a huge tree (sometimes known as Yggdrasil the World Ash), and they have reached their destination by climbing, sometimes with the help of creatures known as the Ratatosk.
Still others have described the Astral Plane is an open, weightless plane with connections to all other planes. It is an empty, mostly barren expanse of nothingness, broken only by shards of matter from other planes and portals leading to new dimensions. Spells such as astral projection and gate access the Astral Plane.

Geography

The Astral Sea, or Astral Plane, connect the Outer Planes. Envisioned as a Great Wheel, the Outer Planes seem to be aligned based on the concepts of good and evil, law and chaos, with similarly aligned planes adjacent to one another, if such a thing is possible.
The "Center" of the Great Wheel is the Outlands, which includes the Fugue Plane of Kelemvor, Cynosure and the Dragon's Eyrie. Acting as a "sun" for the Outlands is the realm of Heliopolis, said to be the dominion of the gods of Mulhorand.
The other Outer Planes include (with their alignments):

• Mount Celestia, The Seven Heavens (LG)

• Bytopia, the Twin Paradises (LG, NG)

• Elysium, the Blessed Fields (NG)

• The Beastlands, The Wildlands (NG, CG)

• Arborea, the Pristine Glades (CG)

• Gladsheim, the Heroic Domains (CG, CN)

• Limbo, the Ever-Changing Chaos (CN)

• Pandemonium, the Windswept Depths (CN, CE)

• The Abyss, The Infinite Layers (CE)

• Tarterus, the Depths (CE, NE)

• Hades, the Gray Wastes (NE)

• Gehenna, The Bleak Eternity (NE, LE)

• The Nine Hells (LE)

• Archeron, the Infinite Battlefield (LE, LN)

• Nirvana, the Ordered Realms (LN)

• Arcadia, the Peaceable Kingdoms (LN, LG)

Type

Dimensional plane

Included Locations

• Acheron
• Arborea
• Arcadia
• Bytopia, the Twin Paradises
• Elysium, the Blessed Fields
• Gehenna
• Gladsheim, the Heroic Domains
• Hades, The Gray Wastes
• Limbo, the Ever-Changing Chaos
• Mount Celestia, The Seven Heavens
• Nirvana
• Pandemonium
• Tarterus
• The Abyss
• The Beastlands (Happy Hunting Grounds)
• The Nine Hells (Baator)
• The Outlands