The Under Colosseum is a gigantic arena with chains dangling from the roof and hundreds of lights. It shows many Japanese neon signs and icons on the walls, much like The Under does itself. The spectators may watch the ongoing battles in different floor levels that have metal fences that block most of the openings to avoid anyone from moving in on the match.
Cipher AdminVenus once used the Colosseum to distribute Shadow Pokémon to Trainers who won knockout challenges, just as Miror B. did in Pyrite Town. Once Wes defeats Venus and she flees, the Colosseum resumes normal operation, offering cash and TM prizes rather than Shadow Pokémon.