Deep Colosseum (Japanese: ボトムコロシアムBottom Colosseum) is a Colosseum located under The Under. After the main storyline of Pokémon Colosseum has ended, the player can challenge Gonzap at the Snagem Hideout. After his defeat, he grants the player a D-Disk, that will allow access to the Deep Colosseum.
Unlike the other three Colosseums in Story Mode, which feature four rotating sets of four Trainers, the Deep Colosseum incorporates five, although the fifth set will not offer a TM prize. 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. This also allows another chance for the player to snag their Shadow Pokémon if they previously failed to do so. After all of them have been defeated, a final Cipher member, nicknamed the "Deep King", appears, using a ShadowShuckle.
This Colosseum has possibly some of the strongest Trainers and valuable prizes. After all of the Cipher Admins have been defeated once more, the Colosseum can continue to be challenged, with high rewards.
The Deep Colosseum is an unknown depth below The Under. There is a giant hidden automatic door that serves as an entrance. Directly behind it is a lobby similar to the lobbies of the other Colosseums in Orre. The Colosseum itself is a gigantic open area with bleachers for spectators. There is a giant rotating fan directly above the battlefield. The Colosseum itself is surrounded by almost all sides by vast, complicated networks of seemingly infinite pipes of varying sizes. Its distance from the reception desk is unknown. One Worker even claims to have reached the Deep Colosseum by just digging downward by shovel.