In Teenage Mutant Ninja Turtles 2: Battle Nexus , there is no specific item or unlockable officially named "Deep Piece." This term may be a misremembered reference to "Antique Items," which are hidden collectibles found throughout the game that unlock various bonuses. Unlocking Content in Battle Nexus
Finish the stage with the item, then take it to ("2nd Time Around") for appraisal. Teenage Mutant Ninja Turtles 2 Battle Nexus Free
Smash the crates on the roof to find a containing an antique item. In Teenage Mutant Ninja Turtles 2: Battle Nexus