If I remember correctly, there was a statue in Pokemon Diamond that said something about "Dia" who/what is this dia. (It's probably Dialga but it might not be)
It is indeed Dialga. There is a Statue in Eterna City of Dialga/Palkia with a faded text under it describing who they are.
http://bulbapedia.bulbagarden.net/wiki/Eterna_City this has the full text for it