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User is made invulnerable for one turn, then hits the next turn. The user cannot make a move between turns. The user can still be hit by Swift on the invulnerable turn. If the user is fully paralyzed or hits itself in confusion when trying to execute the second turn of this move, it will remain invulnerable until it uses Dig again, but it will be free to choose other moves. This move also has an in-game effect that lets the player return to the entrance of a cave or dungeon when it is used by one of their Pokemon.
Dig is a two-turn move that is not seen in the competitive scene for several reasons; the first is the existence of Earthquake, a 100 Base Power move with perfect accuracy that hits for the same damage in one single turn, thus outclassing Dig entirely. Not to mention that on the charge-up turn nothing stops your opponent from simply switching out to a Pokemon that either resists the move or is even immune to it. In a competitive environment, giving your opponent free turns should be avoided at all costs. For everything stated, there are better options for a Ground-type attack moveslot.
Earthquake is a more powerful Ground-type attack with perfect accuracy and no wait turn before the attack.