The Astral Plane is a metaphysical location that can be accessed by mutants with psychic powers.
The Astral Plane is a place lying somewhere between dream and reality. There is no definite "ground" instead there appears to be a black mist with tiny green lights the covers surface. Everything in the Astral Plane takes on a green hue, whether this is due to the light source of the Astral Plane being green or the air that the light passes through being green it is hard to say. Oliver refers the slopping green hills as the "vast subconscious" implying that the environment of the Astral Plane is warped by the user's thoughts and memories subconsciously. Occasionally, a forest like background can be seen instead of hills.
The Astral Plane can also be shaped with conscious effort by the psychic residing there. Everyday is the same in the Astral Plane and though the mutants there can imagine whatever they wish, nothing will ever be real. The reason for this is that their mind is in the Astral Plane while their body remains in reality--comatose. If a mutant spends too long in the astral plane they risk becoming trapped there. Also, if an individual becomes trapped in the Astral Plane their memories and thoughts will begin to drift due to the lack of human contact.
David Haller's father psychically fought and defeated Amahl Farouk, the Shadow King, on the Astral Plane . Oliver Bird, the co-founder of Summerland, regularly visited the Astral Plane until he was unable to return to the physical world. His body was cryogenically frozen to preserve him until he could return. During one session of memory work, David Haller became trapped in the Astral Plane. Oliver brought Cary Loudermilk into the Astral Plane in order to communicate with him and develop a plan to defeat Farouk.