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.
The cave without ruins
In the anime
Iron Ruins in the anime
The Iron Ruins appear in Steeling Peace of Mind! and Saving the World From Ruins!; however, Registeel is not featured at all. The location seems to be more of a temple to Dialga and Palkia, as evidenced by their statues in front. Team Galactic goes there in search of Spear Pillar, using the combined efforts of their machines, the Spear Key, and energy from the Iron Ruins to scan for its location. After their machines were destroyed, they were only 60% done, and revealed that they had planted bombs in the ruins to destroy Iron Island. However, Riley and his Lucario were able to contain the explosion with their Aura until they could send it to a safe location, and Mars was shocked when her detonator didn't work.