The Metal-eating Balloons are ordinary-looking green balloons filled with a liquid that is highly corrosive to metal. When thrown, one such balloon will explode upon contact and release a green gas. This gas will dissolve any metal down to nothingness.
The Metal-eating Balloons were created by Rotor in preparation for the Doomsday Project. One such balloon was used by Sonic the Hedgehog to destroy one of Dr. Robotnik's Doomsday Pods. Initially, Rotor had a setup to produced twenty-five, but after a Pod's rampage resulted in his workshop being crushed, the Freedom Fighters were left with only three (two, after Sonic destroyed the Pod), limiting their use.