Iron Balls are huge metal spheres found in Pikmin 3. They require 20 Pikmin to push.
There are three of them in Story Mode, two located in the Distant Tundra, and one in the Formidable Oak. The first in the Distant Tundra is in the cave where Brittany crash lands and requires 20 Yellow Pikmin to be raised to push the ball and create an exit. The second is on a ledge near the two Arctic Cannon Larvae and the Whiptongue Bulborb. Throw a captain and 20 Pikmin up to push the ball and unlock the cave which leads to the Vehemoth Phosbat arena. A Citrus Lump and an Astringent Clump are also located here. The third and last Iron Ball is found in the Formidable Oak on a hill between the room with Red Bulborbs and the circular area with bridge fragments leading to the second section of the cave. When pushed, the Iron Ball crashes into a Crystal Wall leading to a data file, a swarm of Shearwigs, and a dirt mound with three of Bomb Rocks.
Be cautious when using them, as they can crush all non Winged or Rock Pikmin that are in the balls path. But, they will kill or damage any enemy that wanders in the path. One of the best examples of this is in Jigsaw Fortress, where iron balls are located inside the walls of the fortress where they can kill the bulborbs and roll around and kill your pikmin when you push them.