The Iceberg Ruins (Japanese: ひょうざんのいせき Iceberg Ruins) is a special room in Mt. Coronet. It is only accessible in Pokémon Platinum, when the player has an event Regigigas in their party. It is called the Iceberg Ruins because it is the home of Regice, the Iceberg Pokémon.
If the player enters the cave without the event Regigigas, the cave is simply part of Mt. Coronet.
The extra cave is located inside Mt. Coronet, near the exit to Route 216. A "statue" of Regice is present. By stepping on the 7-dot pattern on the floor and walking up to the statue, Regice 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.