Ballostros were massive sea monsters that would attach to seagoing vessels in order to find prey on land.
One ballostro found its way into the middle of Metropolis attached to a nuclear submarine, causing much destruction. Booster Gold attempted to fight it unsuccessfully, causing the city's power to go out in the process. Supernova arrived on the scene and teleported the beast away. The nuclear submarine was severely damaged, however, and the ensuing blast claimed Booster's life.
Habitat: Ocean depths