Located in the Twilight Temple cavern. If you swim to the wall south of the temple building in square D2 southwest, the title "Forge Ruins" should display on your screen. There is nothing of note to see here other than a wide, low cave.
It is to be assumed the game developers abandoned the Forge Ruins concept but forgot to remove the location from the game's programming.
Some message board users have reported being given a photo request for this area.