You fly to The Ruins and go into the ladder in the first cave.
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If you solve all 4 puzzles it will unlock the 26 "letter" unown. If you open the 4 back rooms (using an escape rope, flash, water stone, and Ho-Oh), catch all 26 unown, and talk to the researcher in the building it will unlock the ! and ? unown. Catch them and talk to the guy again. This will complete the unown side quest, but that's about it. Nothing really special unless you just really have to have all 28 unown.
The tunnel on top the ruins is accessible.The tunnel is where you catch the ? and ! Unown.
Each letter of Unown has its own room, which means that you cant catch all types of Unown in the same room. But you can catch multiple copies of the same Unown in its room.
You have to catch all 26 unown and talk to the scientist a few times for additional pages.
After collecting all 26 Unown, talk to the scientist and he will let you print out your Unown shapes.