Team Aqua and Team Magma are the antagonistic organizations in Hoenn. Team Aqua seeks to expand the sea, while Team Magma aims to expand the land. Their grunts primarily use Water-type (Aqua) or Fire/Ground-type (Magma) Pokémon, respectively. Their leaders, Archie and Maxie, have stronger teams that you'll face multiple times. Understanding their motives and Pokémon helps in these encounters.
