You have to get Kyogre and then go to the ciuty it is in and got to the chamber.
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In Ruby and Sapphire, you have to fight the respective legendary before the doors open.
its actually the 8th gym so you need 7 badges and then it will unlock
Well, try beating the 8th gym and the elite four. I recommend Visiting and area near Pacifidlog town called sky pillar and catching Rayquaza at level 70.
If you're in the middle of the game, then there's no point trying to get up them. The 8th gym badge will enable you to use the HM move, Waterfall, on the field. Then you can go back and collect all the goodies up the waterfalls.
You might need to open the door to the 8th gym in Pokémon Ruby, Sapphire and Emerald. To do this you first have to awaken Rayquaza.