The Bowser Shell is a special item in the Mario Kart series, it has only appeared in Mario Kart: Double Dash!! for the Nintendo GameCube. Only Bowser, Bowser Jr., King Boo, and Petey Piranha can get this special. It is a gigantic shell that, when released, bounces around the track like a normal Green Shell, toppling any kart that it hits as well as knocking down the rear character, making the racers drop their items and temporarily disabling them from switching.
After Bowser's Shell hits the walls of a course roughly fifteen times, the shell breaks apart. Unlike many other items, the Bowser Shell can resist being destroyed by some other items, and even bounces off another special item should it collide with one.
This item was also going to appear in Mario Kart DS, and the model for it is still in the game.