The Seafloor Cavern (Japanese: 海底洞窟Seabed Cavern) is a location in Hoenn, deep beneath the water of Route 128. After surfacing inside the cave, there is a maze of tunnels to the deepest part, where the super-ancient Pokémon resides.
In Pokémon Ruby and Omega Ruby, the cavern is the home of Groudon, who slumbers at its deepest part; in Pokémon Sapphire, Emerald, and Alpha Sapphire, the cavern is the home of Kyogre, who slumbers at its deepest part (in Pokémon Emerald, Groudon slumbers within the Magma Hideout, deep inside of the caverns of Mt. Chimney).
In Pokémon Ruby, Sapphire, and Emerald, the stolen Submarine Explorer 1 is parked at the bottom of the underwater area underneath the Seafloor Cavern. In Pokémon Omega Ruby and Alpha Sapphire, the stolen Submarine Explorer 1 is parked at the shore in the entrance chamber of the Seafloor Cavern.
In the deepest part, MaxieROR or ArchieSEAS will attempt to control the super-ancient Pokémon using the orb he stole from Mt. Pyre. However, the Pokémon does not respond well, and instead will escape, causing harsh sunlightR/extremely harsh sunlightOR or rainS/heavy rainAS throughout eastern Hoenn; in Emerald, both rain and harsh sunlight appear, and they change after the player takes enough steps (counting actions like using Fly).
The Seafloor Cavern briefly appeared in a vision by Courtney in The Vision. Although the interior of the cavern wasn't shown, the top was depicted as a rocky island protruding from the water. At this rocky island, PrimalGroudon appeared and attacked Team Magma.
It properly appeared in The Cavern, where it was shown that Team Aqua also visited it. Archie, Shelly, and three Team Aqua Grunts went deep inside the cave and found a room where Kyogre was resting. Archie woke it up with the Blue Orb, after which Kyogre transformed into Primal Kyogre and started to go on a rampage throughout the Hoenn region.
The protruding island
In the manga
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