Ships carry everything from oil to cars to food to rubber duckies. They also carry huge amounts of ballast water to increase their stability. When the weight of the water isn’t needed — because the ship has taken on cargo, for instance-it gets discharged. When ships collect the water, anything living in it is sucked up into the bowels of the ship, and when it is discharged, anything that survived the trip has a chance to take up residence in its new home. If conditions are ripe, those species can thrive and even become problems. Ballast water is known to be one of the major sources of invasive aquatic species, such as zebra mussels or Northern Pacific sea stars.