From Bulbapedia, the community-driven Pokémon encyclopedia.
Lagoon Colosseum (Japanese: ラグーンコロシアム Lagoon Colosseum) is a Colosseum in Pokémon Battle Revolution. Unlike other Colosseums, this Colosseum cannot be accessed from the main game, and can only be fought in when holding Wi-Fi Battles, Friend Battles, or when playing DS Multiplayer mode. It is accessible from the start of the game.
This Colosseum is located in a lagoon near the island which Pokétopia is on. It is located on a small arena surrounded by palm trees, on an islet in the middle of a lagoon. Battles here are depicted as being held during the middle of the day.