The Ancient Tomb (こだいづかAncient Mound) is a special location in Pokémon Emerald. It is known to be the home of Registeel.
How to encounter Registeel
To get Registeel, you must open the ruins by completing the Braille puzzle in the Sealed Chamber located at Route 134. Once inside the lobby of the tomb, the player must use Fly (in Ruby or Sapphire) or Flash (in Emerald) in the middle of the room to cause the door leading to Registeel to open.