This is the first of the five Elemental Planes, reached by going up from the top level of the Dungeons of Doom while carrying the Amulet of Yendor.
The Plane of Earth is ineligible to leave bones files. The entire level is no-teleport and has an undiggable floor.
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You arrive on the level at the point marked 'x'; the magic portal to the Plane of Air is randomly located in any of the caverns except the entrance one. On arrival, the Wizard of Yendor will put in an appearance (helpfully carrying a spellbook of digging); the entrance cavern is also occupied by an Elvenking carrying a guaranteed pick-axe, and a minotaur with a guaranteed wand of digging. The remainder of the level contains 27 earth elementals, eight rock trolls, five stone giants, five pit vipers, three barbed devils, three pit fiends, a rock piercer, an umber hulk, a dust vortex, four stone golems, a scorpion, and a minotaur, distributed fairly evenly through the caverns.