It has been discovered that a transcendental traveler, upon magically entering the Ethereal Plane of Atrii, can easily return to the real world via magic curtains specially designed to provide a one-way trip. These Dimension Doors are seen as closed curtains hanging suspended in midair.

It is known that the enchantress Y'Gael had at least four portals that were dimension doors (or similar to them) in her Gurth City, Mizniaport, Accardi-by-the-Sea, and Festeron shops.

SOURCE(S): Beyond Zork