Nothing, really.
Chat with our AI personalities
Nothing happens, except the fact that you have many Unowns. ~Katie
The tunnel on top the ruins is accessible.The tunnel is where you catch the ? and ! Unown.
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.
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.
Assuming you are talking about just the letter Unown, after you catch the A-Z you unlock the room where you can catch the "?" and "!" unown who have a half-closed eye unlike the others. From there...well you helped those scientists complete their studies and if you dedicate a box to them (Like I did lol) you have yourself a pretty cool display