At present, the planes always appear in this order, and are followed by the Astral Plane. 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). 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, move forward without the real Amulet of Yendor, or to return to any of the earlier dungeon branches. All of the planes are no-teleport and have undiggable floors. A cursed potion of gain level will not work either, although it can be used to go from dungeon level 1 to the Plane of Earth with the amulet.