Iron ball
The Iron Balls are balls found throughout Pikmin 3 that serve as interactable balls, much like the Paper Balls. They are very plain metallic-grey balls with "20 weight" inscribed on them, such as with the one found at the Yellow Bag in the Distant Tundra.
Iron Balls are always placed at the tip of a slope, so they will rush down a fixed path once the Pikmin have set one in motion. With its momentum and weight, it will break open balls the Pikmin could not have torn down themselves. This opens entrances and exits to other parts of an area. Creatures in the ball's path will be killed, allowing the player to defeat enemies in some rare cases, but Pikmin are more likely to be crushed if the player does not pay attention. Rock Pikmin will survive the hit.
Three Iron Balls are found in the story mode: two in the Distant Tundra, and another in the Formidable Oak. Two more are found in the Formidable Oak Remix in Battle enemies! in Mission Mode.