it is in sootopolis. however, you can only get in after groudon is awakened.
sootopolis
in sootopolis
the cave of origin is only in ruby and sapphire and emerald.
In sootopolis's Cave of Origin
Sootopolis City
sootiplas city
Sootopolis
In the Cave of Origin.
Groudon is in the Cave of Origin, in Sootopolis City. Only in Ruby version. Kyogre is in the Cave of Origin in Sootopolis City, in Sapphire only.
in the cave of origin
It is found in the cave of origin.
In Pokemon Ruby and Sapphire there is no battle between Kyogre and Groudon. That is only in Emerald version. Instead the legendary pokemon (in your case Groudon) is inside the cave of origin in Sootopolis City.