Accessing the Planes
The Plane of Earth is entered by climbing the upstair on dungeon level 1 with the Amulet of Yendor in your possession. If you do not have the Amulet—perhaps because you have been fooled by a fake—you will instead "escape the dungeon", and the game will end immediately, so beware. A cursed potion of gain level is a foolproof way to test if your Amulet is real, as you will only "feel uneasy" with no effect at all if you quaff it while holding a fake Amulet. Once you enter the Planes successfully, it is impossible to return to any of the earlier dungeon branches.
Movement from one plane to the next, and from the Plane of Water to the Astral Plane, then requires finding a magic portal. It is impossible to move backwards to an earlier plane, or to move forward without the real Amulet of Yendor.