Master Sergeant MacDonald James "Mack" Gerhardt (United States Army, callsign: Dirt Diver) is a fictional character on the CBS television series The Unit. He is portrayed by Max Martini. Is a former Army Ranger, air assault trooper with the 101st Airborne and a senior Unit operator. He holds the rank of Master Sergeant (E-8). His wife, Tiffy, and he have two daughters.