You’ll get TM28 Dig from an NPC at the dig site building on Route 4. The dig site is located just before you’d enter the Desert Resort.
Route 4 is right in the middle of Nimbasa City and Castelia City, and it can be reached from either city. The route has sandstorm conditions, which will slowly eat away at the HP of your Pokémon that are not Steel-, Rock-, or Ground-type.
As for the move itself:
Dig is a Ground-type move with perfect Accuracy and 80 base Power. It is a two turn attack that damages the target the turn after the user burrows underground. While underground, the user cannot be affected by most moves.
Outside of battle, Dig can be used to return to the entrance of caves and dungeons. The move can essentially be used as an infinite Escape Rope.
TM28 Dig Location (Step-by-Step)
Step 1: Starting at the Nimbasa City Pokémon Center, head east through the city.
Step 2: Continue east through the city, passing by the Battle Subway.
Step 3: After passing the Battle Subway, turn south and exit the city through the green tunnel.
Step 4: This green tunnel leads to Route 4. Once you’re out of the tunnel, head south onto the route.
Step 5: Continue walking south here, passing the hiking trainer.
Step 6: Above Route 4 is a bridge overpass. Continue south under the overpass, and then turn west onto the next road.
Step 7: Keep west on this road.
Step 8: Once the road ends west, enter the small building over to the left of the vending machine.
Step 9: Once you’re in the building, you’ll see two NPCs in mining hats.
Speak to the NPC standing at the desk on the right-hand side and he’ll give you TM28 Dig.
TM28 Dig Details + Uses
|Dig Move Details|
Dig is one of the few moves in the game that provides the user with a semi-invulnerable turn.
This means that once Dig is used, the user gets one turn of cover where most moves won’t affect it. Then on the second turn, Dig damages the target at 80 base Power.
This can be great for a single-player playthrough of Black & White, since Dig can be used effectively against NPCs who won’t be able to defend against it.
But Dig is useless in competitive play, since any opponent will have ample time to defend against the attack with a move like Protect.
Many Ground-type attacks don’t do much damage anyways. So if you’re playing B&W just for fun or semi-casually, then it’s worth teaching Dig to the best Ground-type Pokémon on your team.
Tip: Pair the move with a strong Physical attacker like Rhyperior or Landorus to get the most bang for your buck.
Dig also has some use outside of battle as well.
When used inside a cave or dungeon, Dig will return you outside to the entrance. It can definitely help you out of a bind, and save you some Poké Dollars on Escape Ropes.