The Iron Ruins (Japanese: くろがねのいせきIron Ruins) is a special room in Iron Island. It is only accessible in Pokémon Platinum, when the player has an eventRegigigas in their party. It is called the Iron Ruins because it is the home of Registeel, the Iron Pokémon.
The extra cave is by the exit, and replaces the cave from Diamond and Pearl. A "statue" of Registeel is present. By stepping on the 7-dot pattern on the floor and walking up to the statue, Registeel will attack. It is at level 30. It can be rebattled multiple times as long as the Event Regigigas is in the party; however, it can only be caught once.
A colored background means that the Pokémon can be found in this location in the specified game. A white background with a colored letter means that the Pokémon cannot be found here.