follow these steps to get all 26 Unknown and the 2 special Unknown.
Step1: go to solacean town and move to the top of town.
Step2: keep going right and go inside the cave.
Step 3 go down any starcase. (hint: there are at least 5 levels and level 1 holds the first few letters of unknown, level 2 has more letters and so on)
Step4: just walk around (or use sweet scent) to encounter the Unknown's.
Step5: when you have collected all 26 Unown's get out of the cave and fly to Veilstone City (the one where you vs the 3rd gym leader) go south of Vielstone City and on the left side of the route, there should be a cave with the rune maniac.
Step6: the rune maniac has made a long tunnel. go all the way to the left and enter the small cavern.
Step7: the 2 special Unown (Unown '?' and Unown '!') are in that small cavern.
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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.
A lot of numbers and or letters
In Diamond/Pearl/Platinum, after you have caught all the Unown the Maniac Tunnel on Route 214 will be fully accessible, now you can catch the "?" and "!" Unown.
After collecting all 26 Unown, talk to the scientist and he will let you print out your Unown shapes.
The tunnel on top the ruins is accessible.The tunnel is where you catch the ? and ! Unown.