Located to the north of Route 108, Sea Mauville was once a high-tech base where scientists and engineers dredged for natural resources beneath the water's surface. The facility was abandoned once it became evident that it was slowly falling over into the waves. Once slated for demolition, the half-sunken structure now exists as a refuge for water-dwelling Pokémon, and a popular tourist attraction.
Surf up to the facility and climb out of the water onto the sloped walkway. Upon doing so, the camera angle rotates 90° counter-clockwise to face westward. A guide stands at the corner to tell visitors about the facility. The Scientist here works for Captain Stern, and is looking for a certain item at the captain's request. He's searched all but the submerged parts of the facility and hasn't yet found it. Collect the nearby Escape Rope, and TM18 (Rain Dance) on the farthest stairway. Enter the interior through the doorway near the Scientist.
Boxes and debris prevent you from reaching the north (right) hallway from here, so take the south (left) hallway instead. The two doors to the left are locked, and the room to the right only has two Trainers, Young Couple Lois & Hal. Walk to the water's edge to grab a Max Repel, then sail out across the water and dive beneath the surface.
Go east (down) to reach a stairway leading to the control room.
Inspect the area near the bottom of the lower-right stairway to find something glimmering faintly in the water. Pick it up to reveal the Scanner.
Take the northwest (upper-right) hallway and return to the surface to reach the far side of the facility.
Climb out of the water and follow the hallway around the corner to collect a White Herb near the entrance. Talk to the female Tuber nearby to receive the Key to Room 1; which opens the upper-left door in the south (left) hallway.
An Uneasy Feeling
Visit the lower-left room on the way back to the submerged section. There seems to be nothing here, but you feel as if you're being watched. Save the game, then search the bookshelf in the room's top-left corner to find an old report about the facility's history. After reading this, you get the same feeling again. Check your party or open your Bag so that the top screen is covered by a menu. Close the menu to find that an Odd Keystone has suddenly appeared behind you!
Room 1 contains Mysterious Sisters Scall & Ion, as well as a Dive Ball. Examine the filing cabinet on the left side of the room to obtain the Key to Room 2, which opens the lower-left door in this same hallway.
Room 2 contains a Revive and two Teammates. Speak to the girl on the right to receive the Key to Room 6, which opens the upper-right door in the north (right) hallway.
Room 6 holds nothing of interest aside from an exterior door on the far side of the room.
Surf to the isle to the north (right) of the facility. Ask the Fisherman here if he's caught anything, and he gives you the Key to Room 4. This opens the upper-left room in the north (right) hallway.
Return to the interior of the facility and open the door to Room 4. Inside, you find the Storage Key, which opens an exterior door.
After passing through Room 6 again, Surf west (up) from the Fisherman to reach a door marked, Storage. The room holds a dozen items scattered across the floor. A Street Thug enters just after you, and intends to make off with these treasures. Suddenly, an old man, who the Street Thug calls Uncle Raikoh, enters. It seems at first that both men are after the goods, but the old man is only interested in a Skitty doll that he calls Takao. Once the two men leave, you are free to collect the Luxury Ball, four Nuggets, eight Big Nuggets, and TM13 (Ice Beam). A sparkling spot in the lower-left corner denotes a Mega Stone; Surf around the boxes to reach the Beedrillite.
Hunting a Legend
Talk to the Scientist again, and he asks that you deliver the Scanner to Captain Stern back in Slateport. Hand it off to Stern in the harbor, and he rewards you with the Clear BellOR/Tidal BellAS, which features in some old legends in the far-off Johto region. Stern says he would sometimes see it glow when visiting Sea Mauville.
Climb the slanted deck of the facility and the Clear Bell in your Bag begins to glow. Head up the incline to the platform's edge, then walk along one of the metal beams protruding over the water. A mysterious ring floats at the beam's end. Save the game, then examine this ring to reveal the legendary Ho-Oh! If you fail to catch it, Ho-Oh will reappear once you enter the Hall of Fame.
Enter the facility and dive down to the underwater control room, and the Tidal Bell in your Bag begins to glow. Go around the stairway to find a mysterious ring floating in the water near the back wall. Save the game, then examine this ring to reveal the legendary Lugia! If you fail to catch it, Lugia will reappear once you enter the Hall of Fame.
Climb out of the water to reach TM11 (Sunny Day), and take the northeast stairs to the basement.
The second floor is darker than the first. Go north to get a Full Heal, then east between the two ponds. In the southeast, you run into Flannery, who is also looking for the rare Pokémon. She's not quite up to the task, and heads back to Lavaridge to better prepare herself. Take the nearby stairs down to the next level.
B2F is shaped like two large rings, with an upper level and lower level. Collect the hidden Nugget on the lower level's east side, and the hidden Super Potion on the upper level's west side. Go down the southern of the two western stairways, then head south to find a Charizardite Y. Climb up the northern of the two western stairways, then down the northwest stairway. Push the boulder into place to create a shortcut, then go down the stairs to the north.
The oppressive heat of the final floor means simply breathing is difficult. Save the game and examine the mysterious ring floating near the north wall to reveal the legendary Heatran! If you fail to catch it, Heatran will reappear once you enter the Hall of Fame.
There are conditional Mirage spots to collect the Legendaries from Johto, Sinnoh, and Unova that cannot be found elsewhere. These locations will only appear after meeting certain conditions. Like with other Legendary Pokémon, if you fail to catch these creatures, they will not reappear until after you enter the Hall of Fame. See the Soaring Legendaries guide for battle coverage for these specific Mirage Spots.