Asclepius

“The hollow rotundity of the world, which has the shape of a sphere, is itself invisible in its totality, by reason of its quality and its form. Choose any place whatsoever at the top of the sphere to look down below, from there you cannot see what is at the bottom. Because of this many believe the world to have the appearance and quality of a place. For it is only because of the forms of individual things whose images seem to be imprinted on the sphere that one believes it in a way to be visible, since it appears to be like a painting. But in reality it is in itself ever invisible. Hence it comes about that the bottom of the sphere, if it be a part of or place in the sphere, is called in Greek Hades, for idein in Greek means to see, and the bottom of the sphere cannot be seen. Hence also ideas are called forms, because they may be seen as forms. Therefore, too, the infernal regions are called in Greek Hades, because they cannot be seen, and in Latin Inferi, because they are at the bottom of the sphere.”
― Asclepius, Asclepius: The Perfect Discourse of Hermes Trismegistus