It's in Sootopolis City. To get there, you need Dive and Surf. Go to Mossdeep then surf south and west untill you hit a big island with a dark ring of water. Use Dive and go all the way to the bottom of the island, you will see a cave. Go in and go up, you will be in Sootopolis City.
Go to the left side of the city and follow the stairs all the way around until you get to the Cave.
In order to enter the cave of origin in Pokémon Emerald, you need to first defeat Team Magma. You also need to have the HM dive.
You must complete the events starting at the cave of origin.
In the cave of origin and marine cave.
There is no Pokemon in the Cave of Origin in Pokemon Emerald.
the cave of origin is only in ruby and sapphire and emerald.
Nothing exciting happens there on emerald.
A desert I think
KYOGRE!
in sootopolis city
The cool thing about emerald is that you can get 2 kyogres in the game: One in the cave of origin the other in marine cave.
In order to find the Cave of Origine in Pokemon Emerald you go to the town square and go east. you will find a field with a tree in it. near the tree cast out your onix and have him dig. when he gets a good hole going jump in it. you will be in the crystal cavern. go under the waterfall at the end of the cavern to enter the Cave of Origin.
In Sootopolis City, at the side where the PokeMart is. Go up, and you'll find Cave of Origin