if saphire, pwn kyorge and the doors should open. if ruby, pwn groudon and doors should open. if emerald, wake up rayquaza to get him to stop kyorge and groudon from fightin.
You can find Steven near the gym is Sootoplis City. When you find him, he will take you to the cave of origin.
you cant
in sootoplis
Go to underwater cave and bet MAXIE then go to the cave. Next you catch GROUDON talk to STEVEN in sootopolis gym area and he let you in.
Go to sootoplis city to battle Gorudon and beat the last gym leader.
It's only locked until you have confronted Kyogre inside the Cave of Origion where you have only 1 chance of capturing it. After that, then you can head to the gym once you have talked with Steven who will be in front of the Gym then you can go inside and battle the Gym Leader Wallace the Master of Water.
In the cave of origin which is in sootoplis city.
at either mossdeep city,sootoplis or ever grande city.
in the cave in sootoplis city when your going to fight groundon
you can get it [rarely] while using dive in the grass around sootoplis city or fishing around the big city
Don't you mean "How to get to Sootoplis City in Pokemon Ruby?" Oh it's simple, get dive and dive underwater near Mosdeep City and find the path, then keep going down until you find a cave, go in the underwater cave and then dive again and there you are!
You need waterfall, which can be obtained in sootoplis's cave origin. Or get a goldeen or seaking.