you go underground and tap the bottom screen on the top screen there is golden little lights. go to the lights that are on the walls, those are places to dig for treasure, sparkles on the floor are traps. once you go to the sparkle go up to where it is on the wall it will say that the wall is bulging and ask if you want to dig. then you will be able to dig.
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You can get the TM28 (Dig) on Route 214 in the Ruin Maniac Cave in Pokemon Diamond.
you can buy it in veilstone
well, dig isn't a HM in Pokemon diamond, but it is an TM. anyway, you can find it on route 214, inside a cave where you'll find the ruin maniac- the same place where you find hippopotas.
Ruin Maniac Cave
you have to go underground and dig for him