Ruby/Sapphire - The Regindaries
So you've captured Groudon or Kyogre, grabbed onto Latias or Latios, and even obtained Rayquaza. What now? You think you're done with all the legendaries. Sure, there's Jirachi and Deoxys, but you can't get them now. Tell you what: you still have three more legendaries to catch. They are the Regi!
There are three Regi Pokemon: Regirock, Regice, and Registeel. They are weird looking legendaries which take a lot of effort to catch. To try to catch them, you must first get two Pokemon by the names of Relicanth and Wailord. Relicanth can be found by diving near Sootopolis City. You can either fish for a Wailord on Route 129 (very rare) or you can evolve it from a Wailmer, which can be found on Route 122 by fishing.
Surf west from Pacifidlog Town, in the Ocean Currents. Before you do, make sure one of your Pokemon knows Dig. Find a spot to dive into the depths, and dive your way to some inscription on a Braille. Emerge back, and you'll find yourself in a cave filled with Brailles. Go to the middle slate of the Braille and use Dig, which should open it. Go into the next room, and put Relicanth as your first Pokemon, and Wailord as your last. Press "A" against the slate, and there'll be an earthquake. A message will appear saying that a door opened somewhere. The three Regi has been released. Now you're all set to catch them!
Fly to Fallarbor Town, and go east into the desert where you found the fossils. You should find a large rock with an entrance. Go in there and go to the middle of the Braille slate. Move it to the right twice, and then down twice. A door will appear. Go inside to get to Regirock.
For Regice, go to Petalburg City, and surf west. Keep on surfing until you reach a small island. Go west and keep surfing up. You'll then find yourself on an island with a small mountain. Go inside, stand in the middle of the Braille wall, and press "A". Wait for a while (about one minute), and a door will appear that leads to Regice.
Registeel is the most powerful of the three. Before trying to catch it, make sure that you have a Mach Bike and a Pokemon with Fly. Go west of the Safari Zone until you reach a mound of earth. Using the Mach Bike, go up, and it will start to rain. You'll find a cave. Go in and Press "A" against the Braille slate. Go back to the middle of the cave, and use Fly. You will then be taken to the cave with Registeel.