Use the move secret power on it.
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In Ruby and Sapphire, you have to fight the respective legendary before the doors open.
I don't think you can, in emerald you need the nationaldex and POW! it's open but in ruby and sapphire you can't.
by getting a life
well they are secret bases but you need to teach one of ur Pokemon secret power(tm43), and u can get it in slateport city at the open market!(In Pokemon Sapphire).
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.