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Big Steelies (also referred to as "bowling balls" in Mario Mania Player's Guide) are large iron balls that appear in Super Mario World in the last battle with Bowser. He drops these from his Koopa Clown Car. Mario can jump over them to avoid getting hit. Mario can also spin jump on them without getting hurt. It can be destroyed if Mario uses his sliding cape attack against it using a Cape Feather but it will show a glitched sprite.
In the Super Mario World episode "Send in the Clown", King Koopa attempts to defeat Mario in one scene by dropping a Big Steely on him, which Mario dodges. King Koopa, with the same method, is shown dropping a Big Steely after Yoshi hits him with his Mechakoopas. Mario, however, uses a golf club and knocks the Big Steely at him.
Big Steelies are mentioned in Super Smash Bros. Melee in the Koopa Clown Car trophy, once again labelled as "bowling balls".
In Mario & Luigi: Bowser's Inside Story, Big Steelies, referred to generically as "iron balls," appear in multiple locations. In Cavi Cape, Bowser must punch a falling one in order to knock it into a button. Later, Midbus crushes Bowser with a massive one, triggering the fight between Giant Bowser and Bowser's Castle. There is also a Hammer called the Iron Ball Hammer, which sometimes causes one to fall onto a random enemy after an "Excellent" rating is achieved.