How do you get into that big rock quite near Fortree City/Do you need to..

It's one of three, theres one in the desert area, and one in the water below the route west of petalburg, it's where the regis are, registeel, regirock, regice, you need to go to paficinglog town, go in the currents, keep goin down till you get to this area of shallow water that your on, I think if you go to the top, go down two/three, the currents will take you to this little small area that you can dive, you might go by it, but you dive in, to through till you see a sign w/ a bundle of dots, it says dive here, so you go back up, and your in a cave, you go up, till you see a pile of rocks, the dots on it says dig here, so thats what you do, then inside, you need a waillord in your first slot, and the rare rock fish recathilth(spelling) in your last slot, and go to the top thing and look a the dots when you have this, the cave will shake and you should hear three doors open(those rock things opening up) if this doesnt happen switch waillord to last slot and the other one to first, but then you go out and get them, they also have riddles, the one your talking about you use fly in the middle of the room and the door will open, the one in water you look at the sign, keep looking at it and wait, after a minute or so it should open, the last one in the desert, you need to go I believe from the sign, down, down, right, right, then use strength, if doesnt work then to the the sign again and go to the left, but I'm sure it's right, and there you go, when you go inside you should see each one there, good luck, there hard to catch, bring 40 ultra balls and have a good party, make sure to save first so you can try again if you mess up

Registeel is inside, You need a Relicanth and Wailord to unlock them.