The Iceberg Ruins is a small cave found in Mt. Coronet. As it is only accessible in Pokémon Platinum, the cave does not exist in Diamond and Pearl. If the player has an event Regigigas in their party, a wild Regice can be encountered by interacting with the statue at the back of the cave.
The cave is located in Mt. cornet, near the exit on Route 216. A "statue" of Regice is present. By stepping on the 7-dot pattern on the floor and walking up to the statue with the event Regigigas in the party, Regice will attack at level 30. It can be rebattled as long as the event Regigias is in your party and it has not been captured.