From Bulbapedia, the community-driven Pokémon encyclopedia.

Deep Colosseum is a Colosseum located under The Under. After the main storyline of Pokémon Colosseum has ended, Miror B. is spotted here, and Wes gains access to it.

The finale of each challenge of this Colosseum is a Cipher Admin, starting with Miror B., then going in the order they were defeated during the course of the game: Dakim, Venus, Ein. After all of them have been defeated, a final Cipher member appears, who has a Shadow Shuckle.

Deep Colosseum can only be accessed using a D-Disk. After all of the Cipher admins have been defeated once more, the Colosseum can continue to be challenged, with high rewards.

Orre
Cities:	Phenac City | Pyrite Town | The Under | Agate Village | Gateon Port
Landmarks:	Relic Stone | The Under Subway | S.S. Libra | Citadark Isle | Kaminko's Lab | Snagem Hideout | Cipher Key Lair | ONBS
Colosseums:	Deep Colosseum | Orre Colosseum | Phenac Stadium | Pre Gym | Pyrite Colosseum | Tower Colosseum | Under Colosseum
Tournament areas:	Mt. Battle | Realgam Tower