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An ancient temple on Monsu Island
Monsu Island (Japanese: モンス島 Mons Island) is an anime-exclusive location in the Hoenn region, found between Lilycove City and Mossdeep City. It appeared in Gaining Groudon and The Scuffle of Legends.
The island's steep and rocky shoreline makes it inaccessible, though aircraft can overcome this issue. Monsu Island is home to an ancient seaside temple that is accessible through a walkway of pillars and a staircase.
After they were all blasted off by Pikachu's Electric attack, Team Rocket found themselves on Monsu Island, while Ash and his friends unknowingly found Team Magma's submarine at the same location. Team Aqua and Team Magma later met on Monsu Island to exchange a war of words.
In The Scuffle of Legends, Archie unleashed Kyogre's full power with the Red Orb, while Pikachu, who was bonded with the Blue Orb, helped control Groudon. The two Legendary Pokémon clashed, and Groudon's arrival caused Monsu Island's volcano to erupt and expand the island's landmass. The pair were eventually quelled with the help of Ash, his friends, and Lance. Kyogre returned to the ocean depths while Groudon escaped into Monsu's island volcano.
Pokémon seen on Monsu Island