The Crypt of the Implementors is the interior chamber of the Tomb of the Unknown Implementor in Hades. It is made of the finest marble and large enough for the headless corpses of four fallen Implementors of uncertain identity (their heads are mounted on poles in the tomb). Though large and esthetically pleasing, the marble crypt is otherwise empty; the sarcophagi, bodies, and rich treasure to be expected in a tomb of this magnificence are missing. Inscribed on one wall is the motto of the implementers, "Feel Free". 

Some have suggested that there is a hidden door in this crypt, used by the Dungeon Master as an alternate route to quickly venture between the first and third levels of the Dungeon of Zork.

SOURCE(S): Dungeon