The Astral Plane is a metaphysical location that can be accessed by mutants with psychic powers. It can be shaped to the whims of the mutant residing there, but remaining too long on the Astral Plane can result in the individual being trapped and unable to return to their physical body.
David Haller's father psychically fought and defeated 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.