Mossdeep City stands perched on an island near Hoenn's eastern edge, where the winds are mild and the weather conditions are stable. This makes it the perfect place for launching rockets, and a perfect home for the Mossdeep Space Center. The seventh Gym in the Hoenn League, run by twin Gym Leaders, is located on the north side of the island, and Steven's house can be found on the island's northwest corner.
Three Hex Maniac sisters can be found in the southeastern-most house. The one in the middle has a special power. By viewing any of your Badges, she is able to recall which Pokémon were in your party when you won that particular Badge.
Visit the house nearest the stairway to the Space Center to meet a young woman who uses a Wingull to send letters to a friend in Fortree City. Talk to the Wingull and it will take flight, off to deliver another note. If you return to Fortree and visit the Pokémon Breeder in the northeast house there, he will give you a Mental Herb that his Wingull found in its travels.
Score the Super Rod
The house nearest the Space Center's launchpad belongs to a lone Fisherman. Speak to him to receive the Super Rod, a high-end fishing rod that can reel in all kinds of sea life.
Mossdeep Space Center
The Mossdeep Space Center stands on a hill in the northeast. Check out the exhibits inside, and take a seat near the windows for some interesting views. A Sailor near the viewing area will give you a Sun Stone he found on the beach.
Mossdeep City Pokémon Gym Leaders: Liza & Tate
The mystic combination!
The Mossdeep Gym specializes in Psychic-type Pokémon, which are vulnerable to Bug-, Dark-, and Ghost-type moves. Dark Pokémon are both immune to Psychic moves and able to inflict super-effective damage. Avoid using Fighting- and Poison-type Pokémon. The Gym is an eerie place, with several platforms floating over a seemingly endless void, connected by strings of energy. Each platform has a Trainer, and most platforms also feature a floating stone that must be activated to alter the direction of these energy trails and reach the next platform.
From the entrance, take the energy trail to the yellow southwest platform and Psychic Preston. Interact with the floating rock, then take the south trail back to the entrance. Take the northern trail again, this time to the orange southeast platform and Psychic Joshua. Activate the floating rock, then take the eastern trail to the green northeast platform and Psychic Fritz. After activating the rock here, you may optionally decide to take the northwest path to battle Hex Maniac Kindra on a smaller nearby platform; otherwise, take the western path to the gray northwest platform and Hex Maniac Patricia. Activating the rock here will open the path to the Gym Leaders, but you must return to the entrance first. From here, either detour to the southeast to fight the final Gym Trainer, Psychic Virgil, or simply head south to the yellow platform again. Head south again to the entrance, then north to reach the Gym Leaders' platform.
Twins Liza and Tate send out two Pokémon, drawing challengers into a Double Battle. Both of their Pokémon are weakened by Water moves; by using Surf, you can attack both at once and bring the battle to a quick end. Otherwise, you may want to target one Pokémon at a time in what would essentially become a two-on-one battle.
After the battle, Liza and Tate award you the Mind Badge, which ensures obedience from all Pokémon up to level 80 and enables the use of Dive in the field. The twins also give you TM04 (Calm Mind) as a prize. Afterwards, step on the glowing panel to warp to the entrance.
A great rumble rocks the city when you step outside, and a giant pillar of green and yellow light is seen bursting up from the sea to the south. Spotting you through the crowd of terrified residents, Steven races over and asks you to his home. Inside, he confirms that Team MagmaOR/Team AquaAS has successfully breached the Seafloor Cavern. The cave's entrance was sealed since ancient times by the power of a certain Pokémon; with the seal broken, the team aims to revive the super-ancient Pokémon inside and make its power their own. Steven resolves to do everything he can to quell the situation, but admits there is a limit to what he can do alone, and asks for your help. He entrusts you with HM07 (Dive) so you can reach the submarine cavern, as well as the Devon Scuba Gear to ensure your safety underwater. He then encourages you to visit Route 128 and pursue the villains inside the cavern.
Connected to the ocean, the appearance of Shoal Cave changes with the tides. Seawater floods the cave during high tide, closing off some areas and opening others. It then drains away during low tide, revealing new areas for exploration, some of which require the Mach Bike to reach. The tides change every six hours, at 3:00 and 9:00.
An old man in the entry chamber offers to make a Shell Bell for your Pokémon. But first, he needs you to collect some of the cave's natural resources: Shoal Shells and Shoal Salts. These items are replenished each day, so there's no limit to the number you can collect. Once you hand over four of each item, he crafts the health-restoring item for you. Surf northwest and take the doorway there to reach the next chamber.
Surf along the water in a clockwise direction, collecting the four Shoal Shells and Rare Candy on the way. Take the southeast doorway back to the entrance, where you can collect a Big Pearl on the nearby ledge.
An old man in the entry chamber offers to make a Shell Bell for your Pokémon. But first, he needs you to collect some of the cave's natural resources: Shoal Shells and Shoal Salts. These items are replenished each day, so there's no limit to the number you can collect. Once you hand over four of each item, he crafts the health-restoring item for you. Head through the north doorway to reach the next chamber.
Head up the stairway, go north and climb down the east-central ladder.
Grab the Shoal Salt and Ice Heal, then climb the hill to the ladder in the northwest.
Go west then south along the two bridges, then climb down the ladder in the center.
Head east to meet a Black Belt and receive a Focus Band from him. Continue east to find another Shoal Salt, then jump the ledge to the south. Use the Mach Bike to climb the sandy slopes to the east in order to reach TM79 (Frost Breath). Slide down the slope on the other side, then descend the ladder to the south.
The final cavern is permanently encased in ice, and the slick floors hinder travel.
Go south from the ladder, across two ice patches. At the third ice patch, slide west to the nearest snow patch within it.
Slide west to a stalagmite in the center of the ice patch.
Slide south to stop at the wall, then west to another stalagmite.
Slide north to the edge of the ice patch, and a wide stairway ahead.
Slide north to the snow patch near the narrow stairway.
Slide west then south to a stalagmite. Slide east then north to reach the central plateau.