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Heart Gold and Soul Silver: Ruins of Alph
The Ruins of Alph are located southwest of Violet City. It is the only place in Johto to catch Unknown. There are four tile puzzles you must complete in order to catch all of the Unknown, and you must catch all 26 alphabetical Pokemon in order to catch the ! and ? Unknown. Two of the tile puzzles require you to Surf through the northern part of Union Cave in order to reach them. The puzzles are simple and you just have to drag pieces to fill in the image and tap the pieces to rotate them accordingly. The images are Ho-Oh, Aerodactyl, Kabuto, and Omastar.

At the back of each chamber containing a tile puzzle, you will find an inscription written in Unknown which tells you to do an action. After you do what it says a door will open and lead you to a room with items. Here are what the inscriptions tell you to do:
Upper Left: Have Ho-Oh in the front of your party (following you).
Lower Left: Have a Water Stone in your inventory.
Upper Right: Use an Escape Rope.
Lower Right: Use Flash.