Magma Cavern

Final Floor: 23F
Unlock Method: Story Progession
Initial Level: -
Maximum Teammates: 3
Items: Yes
Check Map: Yes
Opponents Able to Join: Yes

You'll first encounter Groudon during the story when you're a Fugitive and, when you first defeat it, you won't recruit it. After completing the game, when you return to visit Groudon you will battle it again, but this time it will undergo Primal Reversion. Defeat it and you'll receive the Volcanic Pit Camp and it'll be recruited