Things You'll Need
Instructions
Make Relicanth the No. 1 Pokemon and Wailord the No. 6 Pokemon in your party.
Fly to Pacifidlog Town and surf west through the currents, using the pokemon with the corresponding Hidden Machine moves. Be careful when choosing where you surf, as you have absolutely no control over where you get to go in these currents. Surfing as far south as possible as it will lead you to a calm spot.
Use Dive on the dark spot in the calm water. Follow the path to the south until you reach a Braille inscription. Use Dive again to resurface into a cave.
Walk north to the back of the cave until you reach the giant Braille inscription. Use Dig directly in front of the inscription to ignite a giant earthquake, opening three hidden doors around the Hoenn region leading to three legendary Pokemons: Regice, Regirock and Registeel.
Exit the cave, use Dive to resurface back into normal waters and Fly to go to either Lilycove or Fortree City.
Head to the log bridge on Route 120. The cave to Registeel will now appear here. Enter the cave and read the Braille message on the wall. Use Flash in the center of the cave (four steps in front of the entrance to the cave). The door to Registeel will now be open in the back of the cave. Save the game before entering the cave to encounter a level 40 Registeel.
Battle Registeel and reduce its health as much as possible without causing it to faint. This is your only chance to catch Registeel. Inflict battle status ailments such as Sleep, Paralyze or Frozen to increase your chance of catching it. Use your Ultra Balls, Great Balls and finally Poke Balls to help defeat Registeel.