From Bulbapedia, the community-driven Pokémon encyclopedia.
The Whirl Cup Colosseum (Japanese: 古代コロシアム Ancient Colosseum), located in Scarlet City on Red Rock Isle in the Whirl Islands area of the Johto region, is a massive stadium where, once every three years, the main battles for the Whirl Cup are held. It first appeared at the end of Octillery the Outcast, and also appeared in the two episodes after that. Maya, the Sea Priestess, resides at the top of the structure, observing all matches. Behind the Colosseum are what appear to be ruins of an ancient culture.
It bears some resemblance to the Roman Colosseum.