Ruin Maniac Cave
you can buy it in veilstone
You can get the TM28 (Dig) on Route 214 in the Ruin Maniac Cave in Pokemon Diamond.
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.
you have to go underground and dig for him
it can if you teach trapinch dig before you evole it