In closed beta, there was a ramp to an underground Old Ironforge, accessed from King Magni’s chamber.  The hallway would lead to a underground chamber and at the end there was a closed gate. The access to Old Ironforge was blocked in a patch during Open Beta.  I heard a player once comment that a developer had said it was there just for looks and there is nothing planned, but … according to Warcraft RPG: Lands of Conflict—page 77—Prospector Brann Bronzebeard comments that Old Ironforge does exist.

Below Ironforge, exists a natural chamber made of organic stone formations.  No chisel or pick has touched the area.  What lies below is the Hall of Thanes, where the past Kings of Iron Forge are buried.  Down there are two important places held sacred by all Dwarves:  The Old Throne and the object for which the Fortress is named for … the Iron Forge.  The enormous artifact is a legacy left behind by the Titan Khaz’Goroth for the Earthen—former stone-skinned race the Dwarves descend from.  The Iron Forge is a relic and is never been used by the Dwarves.

It is kept as a treasure within the Hall of Thanes to honor the dead Kings.  Brann Bronzebeard thinks that the Titans left the anvil behind for some purpose the Dwarves need to find out.  Maybe once the Dwarves uncover the truth of their ancestry, they will find its purpose.