You need to find the submarine that team magma stole.
you need seven badges to enter the cave. to do it, go to sootopolis city and talk to steven who will then lead you to the cave
go to dewford and go in the cave the man in there has it
You don't get to go in there until team magma release Groudon. at which point you get let in by the old man and Steven.
go to the cave of origin in sootopolis city
Go to the right of rustboro city, or the the left of Verdanturf Town. That is the cave you see.
He is in Granite cave on dewford town you give him the letter in the cave.
In Pokemon emerald/Ruby/Sapphire,go to Dewford Town and go to the Cave and ask the man that istanding there and he'll give it to you.
After you beat the Pokemon league, go to One Island. Go to the base of Mt. Ember, where you saw the 2 Rockets. They should now be standing in front of the opening of a cave. Beat them, go in, and go to the end of the cave, where you'll find the ruby.
beat the 2nd gym and go in the cave
go t ceribii net . com
first go to the cave of origin in sootopolis, and go to the end, you will see groudon there
He stays in Evergrande Cave and every now and then you can battle him again