This achievement is earned through completing the three levels that have the Guard Dog for the digging ability.
The levels are:
Port Royal(The Black Pearl-Chapter 1)
Pelegostos(Dead Man's Chest-Chapter 1)
Brethren Court(At World's End-Chapter 4)
If you did not receive the achievement because of playing co-op and having the second player play as the dog at all, just replay story for these three levels until they pop, also complete each level thru too.