The Moon is surrounded by a mist of unknown nature, which is not visible from the surface of the Earth, but which prevents close observation of the surface. Venturing into this mist, called the Veil of Ignorance, causes craft to lose their ability to fly, and instead to fall, presumably causing the death of all crew. Thus the Moon is unexplored, and its nature and inhabitants unknown.
The esoteric book History of the Computer God speaks of cities on the far side of the Moon, or perhaps buried inside it. Some interpret the text as meaning that the Moon is hollow, and that these cities lie on the inside surface.
"Infrared crusader priests from beyond comprehension" are said to travel to the world, steal the "Brain Bank Brain" of their victims, and house them in these cities.
The "Brain Bank Brain" is said to be the "other real brain". The meaning is obscure, but perhaps relates to the ancient Egyptian belief in multiple souls.