Sea Mauville is a decrepit research facility in the Hoenn region located to the north of Route 108. The second part of the structure can only be accessed by using Dive and contains the 
Scanner.
Sea Mauville includes both the decrepit platform itself as well as the water area and sandbar outside of the facility; the northern exterior can only be accessed by travelling underwater inside the platform using Dive.
Sea Mauville takes the place of the Abandoned Ship from Pokémon Ruby, Sapphire, and Emerald.
In Omega Ruby, at the end of the beam that extends a significant distance west and upwards from the platform, a portal appears if the player has the 
Clear Bell. Interacting with this portal will initiate a battle with Ho-Oh. In Alpha Sapphire, at the control desk in the submerged control room, a portal appears if the player has the 
Tidal Bell. Interacting with this portal will initiate a battle with Lugia.
After defeating or capturing Kyogre/Groudon, in the first (southeastern/lower left) room of the northern (right-hand) dry corridor, when the player enters the room the message "It feels as though you're being watched" will be displayed. If the player then stands in the top-left corner of the room and opens a menu such that they cannot see the overworld, upon returning the screen to display the overworld the 
Odd Keystone will appear, enabling the player to battle Spiritomb. The Odd Keystone will also appear if the player saves at that position and loads that save; in this case, this effectively means that the player cannot continue without battling Spiritomb.
