you have to beat the elite 4
You can get the TM28 (Dig) on Route 214 in the Ruin Maniac Cave in Pokemon Diamond.
you have to pass through a part of victory road on the main floor to the west but not before defeating the pokemon league
In Sendoff Springs near Route 214
You can get Gible on Pokemon Diamond in the Wayward Cave hidden under Cycling Road in Route 206. You will need the HM move Strength to access the cave.
You go to route 213 and then go to spring path and then go in the cave that is there.
you go through victory road and find a little cave entrance inside. go inside and keep going until you get outside, that is route 224
it leads to another section of the cave, which exits on route 224
go back to the beginig of the cave
i experienced that but my friend told me where. it's in wayward cave. Route 206.
after you got the national pokedex there will be a new lake cooled turn back cave near route 214.
Go through the cave
you just keep going forward.